diff --git a/components/modals/AddReminderModal.tsx b/components/modals/AddReminderModal.tsx
index fdf378a..e61fdd5 100644
--- a/components/modals/AddReminderModal.tsx
+++ b/components/modals/AddReminderModal.tsx
@@ -1,6 +1,11 @@
 import React, { useState, useEffect, useRef } from 'react';
 import { CustomReminder } from '../../types';
 import { reminderIcons } from '../icons/Icons';
+import {
+  MIN_REMINDER_FREQUENCY_MINUTES,
+  MAX_REMINDER_FREQUENCY_MINUTES,
+  validateReminderInputs,
+} from './reminderValidation';
 
 interface AddReminderModalProps {
   isOpen: boolean;
@@ -19,9 +24,21 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
   const [startTime, setStartTime] = useState('09:00');
   const [endTime, setEndTime] = useState('17:00');
   const [isSaving, setIsSaving] = useState(false);
+  const [errors, setErrors] = useState<{
+    frequency?: string;
+    timeRange?: string;
+  }>({});
 
   const titleInputRef = useRef<HTMLInputElement>(null);
 
+  const validate = () => {
+    return validateReminderInputs({
+      frequencyMinutes,
+      startTime,
+      endTime,
+    });
+  };
+
   useEffect(() => {
     if (isOpen) {
       setTitle('');
@@ -30,14 +47,26 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
       setStartTime('09:00');
       setEndTime('17:00');
       setIsSaving(false);
+      setErrors({});
       setTimeout(() => titleInputRef.current?.focus(), 100);
     }
   }, [isOpen]);
 
+  useEffect(() => {
+    if (!isOpen) return;
+    setErrors(validate());
+  }, [isOpen, frequencyMinutes, startTime, endTime]);
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     if (!title || isSaving) return;
 
+    const validation = validate();
+    setErrors(validation);
+    if (Object.keys(validation).length > 0) {
+      return;
+    }
+
     setIsSaving(true);
     try {
       await onAdd({
@@ -125,11 +154,30 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
                 id='rem-frequency'
                 value={frequencyMinutes}
                 onChange={e =>
-                  setFrequencyMinutes(parseInt(e.target.value, 10))
+                  setFrequencyMinutes(
+                    Number.isNaN(parseInt(e.target.value, 10))
+                      ? 0
+                      : parseInt(e.target.value, 10)
+                  )
                 }
-                min='1'
+                min={MIN_REMINDER_FREQUENCY_MINUTES}
+                max={MAX_REMINDER_FREQUENCY_MINUTES}
+                aria-invalid={Boolean(errors.frequency)}
+                aria-describedby='rem-frequency-help'
                 className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
               />
+              <p
+                id='rem-frequency-help'
+                className={`mt-1 text-sm ${
+                  errors.frequency
+                    ? 'text-red-600 dark:text-red-400'
+                    : 'text-slate-500 dark:text-slate-400'
+                }`}
+              >
+                {errors.frequency
+                  ? errors.frequency
+                  : `Minimum ${MIN_REMINDER_FREQUENCY_MINUTES} minutes, maximum ${MAX_REMINDER_FREQUENCY_MINUTES} minutes.`}
+              </p>
             </div>
 
             <div className='grid grid-cols-2 gap-4'>
@@ -146,6 +194,10 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
                   value={startTime}
                   onChange={e => setStartTime(e.target.value)}
                   required
+                  aria-invalid={Boolean(errors.timeRange)}
+                  aria-describedby={
+                    errors.timeRange ? 'rem-time-help' : undefined
+                  }
                   className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
                 />
               </div>
@@ -162,10 +214,22 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
                   value={endTime}
                   onChange={e => setEndTime(e.target.value)}
                   required
+                  aria-invalid={Boolean(errors.timeRange)}
+                  aria-describedby={
+                    errors.timeRange ? 'rem-time-help' : undefined
+                  }
                   className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
                 />
               </div>
             </div>
+            {errors.timeRange && (
+              <p
+                id='rem-time-help'
+                className='text-sm text-red-600 dark:text-red-400'
+              >
+                {errors.timeRange}
+              </p>
+            )}
           </div>
           <div className='px-6 py-4 bg-slate-50 dark:bg-slate-700/50 flex justify-end space-x-3 rounded-b-lg border-t border-slate-200 dark:border-slate-700'>
             <button
@@ -178,7 +242,9 @@ const AddReminderModal: React.FC<AddReminderModalProps> = ({
             </button>
             <button
               type='submit'
-              disabled={isSaving}
+              disabled={
+                isSaving || Boolean(errors.frequency || errors.timeRange)
+              }
               className='px-4 py-2 text-sm font-medium text-white bg-indigo-600 border border-transparent rounded-md shadow-sm hover:bg-indigo-700 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-500 disabled:opacity-50 disabled:cursor-not-allowed flex items-center dark:focus:ring-offset-slate-800'
             >
               {isSaving ? 'Adding...' : 'Add Reminder'}
diff --git a/components/modals/EditReminderModal.tsx b/components/modals/EditReminderModal.tsx
index d3488e4..286f7bb 100644
--- a/components/modals/EditReminderModal.tsx
+++ b/components/modals/EditReminderModal.tsx
@@ -1,6 +1,11 @@
 import React, { useState, useEffect, useRef } from 'react';
 import { CustomReminder } from '../../types';
 import { reminderIcons } from '../icons/Icons';
+import {
+  MIN_REMINDER_FREQUENCY_MINUTES,
+  MAX_REMINDER_FREQUENCY_MINUTES,
+  validateReminderInputs,
+} from './reminderValidation';
 
 interface EditReminderModalProps {
   isOpen: boolean;
@@ -21,9 +26,20 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
   const [startTime, setStartTime] = useState('09:00');
   const [endTime, setEndTime] = useState('17:00');
   const [isSaving, setIsSaving] = useState(false);
+  const [errors, setErrors] = useState<{
+    frequency?: string;
+    timeRange?: string;
+  }>({});
 
   const titleInputRef = useRef<HTMLInputElement>(null);
 
+  const validate = () =>
+    validateReminderInputs({
+      frequencyMinutes,
+      startTime,
+      endTime,
+    });
+
   useEffect(() => {
     if (isOpen && reminder) {
       setTitle(reminder.title);
@@ -32,14 +48,26 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
       setStartTime(reminder.startTime);
       setEndTime(reminder.endTime);
       setIsSaving(false);
+      setErrors({});
       setTimeout(() => titleInputRef.current?.focus(), 100);
     }
   }, [isOpen, reminder]);
 
+  useEffect(() => {
+    if (!isOpen) return;
+    setErrors(validate());
+  }, [isOpen, frequencyMinutes, startTime, endTime]);
+
   const handleSubmit = async (e: React.FormEvent) => {
     e.preventDefault();
     if (!title || !reminder || isSaving) return;
 
+    const validation = validate();
+    setErrors(validation);
+    if (Object.keys(validation).length > 0) {
+      return;
+    }
+
     setIsSaving(true);
     try {
       await onUpdate({
@@ -126,12 +154,28 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
                 type='number'
                 id='rem-edit-frequency'
                 value={frequencyMinutes}
-                onChange={e =>
-                  setFrequencyMinutes(parseInt(e.target.value, 10))
-                }
-                min='1'
+                onChange={e => {
+                  const value = parseInt(e.target.value, 10);
+                  setFrequencyMinutes(Number.isNaN(value) ? 0 : value);
+                }}
+                min={MIN_REMINDER_FREQUENCY_MINUTES}
+                max={MAX_REMINDER_FREQUENCY_MINUTES}
+                aria-invalid={Boolean(errors.frequency)}
+                aria-describedby='rem-edit-frequency-help'
                 className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
               />
+              <p
+                id='rem-edit-frequency-help'
+                className={`mt-1 text-sm ${
+                  errors.frequency
+                    ? 'text-red-600 dark:text-red-400'
+                    : 'text-slate-500 dark:text-slate-400'
+                }`}
+              >
+                {errors.frequency
+                  ? errors.frequency
+                  : `Minimum ${MIN_REMINDER_FREQUENCY_MINUTES} minutes, maximum ${MAX_REMINDER_FREQUENCY_MINUTES} minutes.`}
+              </p>
             </div>
 
             <div className='grid grid-cols-2 gap-4'>
@@ -148,6 +192,10 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
                   value={startTime}
                   onChange={e => setStartTime(e.target.value)}
                   required
+                  aria-invalid={Boolean(errors.timeRange)}
+                  aria-describedby={
+                    errors.timeRange ? 'rem-edit-time-help' : undefined
+                  }
                   className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
                 />
               </div>
@@ -164,10 +212,22 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
                   value={endTime}
                   onChange={e => setEndTime(e.target.value)}
                   required
+                  aria-invalid={Boolean(errors.timeRange)}
+                  aria-describedby={
+                    errors.timeRange ? 'rem-edit-time-help' : undefined
+                  }
                   className='mt-1 block w-full px-3 py-2 border border-slate-300 rounded-md shadow-sm focus:outline-none focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm bg-white dark:bg-slate-700 dark:border-slate-600'
                 />
               </div>
             </div>
+            {errors.timeRange && (
+              <p
+                id='rem-edit-time-help'
+                className='text-sm text-red-600 dark:text-red-400'
+              >
+                {errors.timeRange}
+              </p>
+            )}
           </div>
           <div className='px-6 py-4 bg-slate-50 dark:bg-slate-700/50 flex justify-end space-x-3 rounded-b-lg border-t border-slate-200 dark:border-slate-700'>
             <button
@@ -180,7 +240,9 @@ const EditReminderModal: React.FC<EditReminderModalProps> = ({
             </button>
             <button
               type='submit'
-              disabled={isSaving}
+              disabled={
+                isSaving || Boolean(errors.frequency || errors.timeRange)
+              }
               className='px-4 py-2 text-sm font-medium text-white bg-indigo-600 border border-transparent rounded-md shadow-sm hover:bg-indigo-700 focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-500 disabled:opacity-50 disabled:cursor-not-allowed flex items-center dark:focus:ring-offset-slate-800'
             >
               {isSaving ? 'Saving...' : 'Save Changes'}
diff --git a/components/modals/__tests__/AddReminderModal.test.tsx b/components/modals/__tests__/AddReminderModal.test.tsx
new file mode 100644
index 0000000..1f42b74
--- /dev/null
+++ b/components/modals/__tests__/AddReminderModal.test.tsx
@@ -0,0 +1,72 @@
+import React from 'react';
+import '@testing-library/jest-dom';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import AddReminderModal from '../AddReminderModal';
+
+describe('AddReminderModal validation', () => {
+  const onClose = jest.fn();
+  const onAdd = jest.fn();
+
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('displays an error when frequency is below the minimum', () => {
+    render(<AddReminderModal isOpen onClose={onClose} onAdd={onAdd} />);
+
+    const frequencyInput = screen.getByLabelText('Remind me every (minutes)');
+    fireEvent.change(frequencyInput, { target: { value: '1' } });
+
+    expect(
+      screen.getByText(/Choose a value between 5 and 720 minutes\./i)
+    ).toBeInTheDocument();
+    const submitButton = screen.getByRole('button', { name: /add reminder/i });
+    expect(submitButton).toBeDisabled();
+  });
+
+  it('disables submit when end time is earlier than start time', () => {
+    render(<AddReminderModal isOpen onClose={onClose} onAdd={onAdd} />);
+
+    const startInput = screen.getByLabelText('From');
+    const endInput = screen.getByLabelText('Until');
+
+    fireEvent.change(startInput, { target: { value: '10:00' } });
+    fireEvent.change(endInput, { target: { value: '09:00' } });
+
+    expect(
+      screen.getByText(/End time must be later than start time\./i)
+    ).toBeInTheDocument();
+    expect(
+      screen.getByRole('button', { name: /add reminder/i })
+    ).toBeDisabled();
+  });
+
+  it('calls onAdd with valid data', async () => {
+    const resolveAdd = jest.fn().mockResolvedValue(undefined);
+    render(<AddReminderModal isOpen onClose={onClose} onAdd={resolveAdd} />);
+
+    fireEvent.change(screen.getByLabelText('Title'), {
+      target: { value: 'Hydrate' },
+    });
+    fireEvent.change(screen.getByLabelText('Remind me every (minutes)'), {
+      target: { value: '30' },
+    });
+    fireEvent.change(screen.getByLabelText('From'), {
+      target: { value: '08:00' },
+    });
+    fireEvent.change(screen.getByLabelText('Until'), {
+      target: { value: '12:00' },
+    });
+
+    fireEvent.click(screen.getByRole('button', { name: /add reminder/i }));
+
+    await waitFor(() => expect(resolveAdd).toHaveBeenCalledTimes(1));
+    expect(resolveAdd).toHaveBeenCalledWith({
+      title: 'Hydrate',
+      icon: 'bell',
+      frequencyMinutes: 30,
+      startTime: '08:00',
+      endTime: '12:00',
+    });
+  });
+});
diff --git a/components/modals/reminderValidation.ts b/components/modals/reminderValidation.ts
new file mode 100644
index 0000000..27fc9fb
--- /dev/null
+++ b/components/modals/reminderValidation.ts
@@ -0,0 +1,54 @@
+export const MIN_REMINDER_FREQUENCY_MINUTES = 5;
+export const MAX_REMINDER_FREQUENCY_MINUTES = 720;
+
+export const parseTimeToMinutes = (time: string): number | null => {
+  const [hours, minutes] = time.split(':').map(Number);
+  if (
+    Number.isNaN(hours) ||
+    Number.isNaN(minutes) ||
+    hours < 0 ||
+    hours > 23 ||
+    minutes < 0 ||
+    minutes > 59
+  ) {
+    return null;
+  }
+
+  return hours * 60 + minutes;
+};
+
+export interface ReminderValidationParams {
+  frequencyMinutes: number;
+  startTime: string;
+  endTime: string;
+}
+
+export const validateReminderInputs = ({
+  frequencyMinutes,
+  startTime,
+  endTime,
+}: ReminderValidationParams): { frequency?: string; timeRange?: string } => {
+  const errors: { frequency?: string; timeRange?: string } = {};
+  const frequency = Number(frequencyMinutes);
+
+  if (
+    !Number.isInteger(frequency) ||
+    frequency < MIN_REMINDER_FREQUENCY_MINUTES ||
+    frequency > MAX_REMINDER_FREQUENCY_MINUTES
+  ) {
+    errors.frequency = `Choose a value between ${MIN_REMINDER_FREQUENCY_MINUTES} and ${MAX_REMINDER_FREQUENCY_MINUTES} minutes.`;
+  }
+
+  const startMinutes = parseTimeToMinutes(startTime);
+  const endMinutes = parseTimeToMinutes(endTime);
+
+  if (
+    startMinutes === null ||
+    endMinutes === null ||
+    endMinutes <= startMinutes
+  ) {
+    errors.timeRange = 'End time must be later than start time.';
+  }
+
+  return errors;
+};