diff --git a/country-explorer/src/components/App.test.tsx b/country-explorer/src/components/App.test.tsx
index ee57e11..9f40a98 100644
--- a/country-explorer/src/components/App.test.tsx
+++ b/country-explorer/src/components/App.test.tsx
@@ -3,6 +3,7 @@ import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 import { getCountriesByRegion } from '../api/countries';
 import { mockCountries } from '../test/fixtures';
+import type { Country } from '../types/country';
 import App from './App';
 
 vi.mock('../api/countries', async (importOriginal) => {
@@ -21,6 +22,76 @@ function renderApp() {
   );
 }
 
+// Spans multiple subregions with population figures that don't follow the
+// alphabetical order, so region filtering and each sort option can be told
+// apart from one another.
+const filterSortCountries: Country[] = [
+  {
+    code: 'DEU',
+    name: 'Germany',
+    capital: 'Berlin',
+    region: 'Europe',
+    subregion: 'Western Europe',
+    population: 20000000,
+    area: 357114,
+    flagUrl: 'https://example.com/flags/de.svg',
+    currencies: ['Euro (EUR)'],
+    languages: ['German'],
+  },
+  {
+    code: 'ITA',
+    name: 'Italy',
+    capital: 'Rome',
+    region: 'Europe',
+    subregion: 'Southern Europe',
+    population: 60000000,
+    area: 301340,
+    flagUrl: 'https://example.com/flags/it.svg',
+    currencies: ['Euro (EUR)'],
+    languages: ['Italian'],
+  },
+  {
+    code: 'NOR',
+    name: 'Norway',
+    capital: 'Oslo',
+    region: 'Europe',
+    subregion: 'Northern Europe',
+    population: 5379475,
+    area: 323802,
+    flagUrl: 'https://example.com/flags/no.svg',
+    currencies: ['Norwegian krone (NOK)'],
+    languages: ['Norwegian'],
+  },
+  {
+    code: 'SWE',
+    name: 'Sweden',
+    capital: 'Stockholm',
+    region: 'Europe',
+    subregion: 'Northern Europe',
+    population: 10353442,
+    area: 450295,
+    flagUrl: 'https://example.com/flags/se.svg',
+    currencies: ['Swedish krona (SEK)'],
+    languages: ['Swedish'],
+  },
+];
+
+// Opens a Dropdown by its current accessible name (label + selected value).
+function openDropdown(name: string) {
+  fireEvent.click(screen.getByRole('button', { name }));
+}
+
+// The selected country is kept across a filter/sort change when it's still
+// in the list, so order-dependent assertions rewind to the first country
+// first rather than assuming what's currently selected.
+function rewindToFirstCountry() {
+  let previous = screen.getByRole('button', { name: 'Previous country' });
+  while (!previous.hasAttribute('disabled')) {
+    fireEvent.click(previous);
+    previous = screen.getByRole('button', { name: 'Previous country' });
+  }
+}
+
 beforeEach(() => {
   window.sessionStorage.clear();
   window.localStorage.clear();
@@ -63,7 +134,8 @@ describe('App navigation', () => {
     renderApp();
     await screen.findByRole('heading', { name: 'Norway' });
 
-    fireEvent.change(screen.getByLabelText('Country'), { target: { value: 'SWE' } });
+    openDropdown('Country Norway');
+    fireEvent.click(screen.getByRole('radio', { name: 'Sweden' }));
 
     expect(screen.getByRole('heading', { name: 'Sweden' })).toBeInTheDocument();
     expect(screen.getByText('Stockholm')).toBeInTheDocument();
@@ -82,3 +154,89 @@ describe('App navigation', () => {
     expect(screen.getByRole('button', { name: 'Next country' })).toBeDisabled();
   });
 });
+
+describe('App filtering and sorting', () => {
+  it('filters the countries down to the selected region', async () => {
+    mockedGetCountriesByRegion.mockResolvedValue(filterSortCountries);
+    renderApp();
+    await screen.findByRole('heading', { name: 'Germany' });
+
+    openDropdown('Region All regions');
+    fireEvent.click(screen.getByRole('radio', { name: 'Northern Europe' }));
+
+    expect(screen.getByRole('heading', { name: 'Norway' })).toBeInTheDocument();
+    expect(screen.getByText('Country 1 of 2')).toBeInTheDocument();
+
+    fireEvent.click(screen.getByRole('button', { name: 'Next country' }));
+
+    expect(screen.getByRole('heading', { name: 'Sweden' })).toBeInTheDocument();
+    expect(screen.getByRole('button', { name: 'Next country' })).toBeDisabled();
+  });
+
+  it('brings every country back into view once "All regions" is selected', async () => {
+    mockedGetCountriesByRegion.mockResolvedValue(filterSortCountries);
+    renderApp();
+    await screen.findByRole('heading', { name: 'Germany' });
+
+    openDropdown('Region All regions');
+    fireEvent.click(screen.getByRole('radio', { name: 'Northern Europe' }));
+    expect(screen.getByRole('heading', { name: 'Norway' })).toBeInTheDocument();
+    expect(screen.getByText('Country 1 of 2')).toBeInTheDocument();
+
+    openDropdown('Region Northern Europe');
+    fireEvent.click(screen.getByRole('radio', { name: 'All regions' }));
+
+    // the previously selected country stays selected since it's still
+    // available, but the full four-country list is back in view
+    expect(screen.getByRole('heading', { name: 'Norway' })).toBeInTheDocument();
+    expect(screen.getByText('Country 3 of 4')).toBeInTheDocument();
+
+    rewindToFirstCountry();
+    expect(screen.getByRole('heading', { name: 'Germany' })).toBeInTheDocument();
+  });
+
+  it('sorts the countries alphabetically by name', async () => {
+    mockedGetCountriesByRegion.mockResolvedValue(filterSortCountries);
+    renderApp();
+    await screen.findByRole('heading', { name: 'Germany' });
+    rewindToFirstCountry();
+
+    const expectedOrder = ['Germany', 'Italy', 'Norway', 'Sweden'];
+    expect(screen.getByRole('heading', { name: expectedOrder[0] })).toBeInTheDocument();
+
+    for (const name of expectedOrder.slice(1)) {
+      fireEvent.click(screen.getByRole('button', { name: 'Next country' }));
+      expect(screen.getByRole('heading', { name })).toBeInTheDocument();
+    }
+    expect(screen.getByRole('button', { name: 'Next country' })).toBeDisabled();
+  });
+
+  it('sorts the countries by population from high to low', async () => {
+    mockedGetCountriesByRegion.mockResolvedValue(filterSortCountries);
+    renderApp();
+    await screen.findByRole('heading', { name: 'Germany' });
+
+    openDropdown('Sort by Name (A–Z)');
+    fireEvent.click(screen.getByRole('radio', { name: 'Population (high to low)' }));
+    rewindToFirstCountry();
+
+    const expectedOrder = ['Italy', 'Germany', 'Sweden', 'Norway'];
+    expect(screen.getByRole('heading', { name: expectedOrder[0] })).toBeInTheDocument();
+
+    for (const name of expectedOrder.slice(1)) {
+      fireEvent.click(screen.getByRole('button', { name: 'Next country' }));
+      expect(screen.getByRole('heading', { name })).toBeInTheDocument();
+    }
+    expect(screen.getByRole('button', { name: 'Next country' })).toBeDisabled();
+  });
+
+  it('shows a message when the selected region has no matching countries', async () => {
+    window.sessionStorage.setItem('country-explorer:selectedRegion', 'Eastern Europe');
+    mockedGetCountriesByRegion.mockResolvedValue(filterSortCountries);
+    renderApp();
+
+    await screen.findByText('No countries match the selected region.');
+
+    expect(screen.queryByRole('heading', { level: 3 })).not.toBeInTheDocument();
+  });
+});
diff --git a/country-explorer/src/utils/sortCountries.test.ts b/country-explorer/src/utils/sortCountries.test.ts
new file mode 100644
index 0000000..5544eaf
--- /dev/null
+++ b/country-explorer/src/utils/sortCountries.test.ts
@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest';
+import type { Country } from '../types/country';
+import { isSortOption, sortCountries } from './sortCountries';
+
+function makeCountry(overrides: Partial<Country>): Country {
+  return {
+    code: 'XXX',
+    name: 'Country',
+    capital: '',
+    region: '',
+    subregion: '',
+    population: null,
+    area: null,
+    flagUrl: '',
+    currencies: [],
+    languages: [],
+    ...overrides,
+  };
+}
+
+const norway = makeCountry({ code: 'NOR', name: 'Norway', population: 5379475 });
+const sweden = makeCountry({ code: 'SWE', name: 'Sweden', population: 10353442 });
+const iceland = makeCountry({ code: 'ISL', name: 'Iceland', population: 372520 });
+const unknownPopulation = makeCountry({ code: 'ATL', name: 'Atlantis', population: null });
+
+describe('sortCountries', () => {
+  it('sorts alphabetically by name for name-asc', () => {
+    const result = sortCountries([sweden, norway, iceland], 'name-asc');
+
+    expect(result.map((country) => country.name)).toEqual(['Iceland', 'Norway', 'Sweden']);
+  });
+
+  it('sorts by population from high to low for population-desc', () => {
+    const result = sortCountries([iceland, sweden, norway], 'population-desc');
+
+    expect(result.map((country) => country.name)).toEqual(['Sweden', 'Norway', 'Iceland']);
+  });
+
+  it('sorts countries with unknown population last', () => {
+    const result = sortCountries([unknownPopulation, sweden, iceland], 'population-desc');
+
+    expect(result.map((country) => country.name)).toEqual(['Sweden', 'Iceland', 'Atlantis']);
+  });
+
+  it('returns an empty array for both sort options', () => {
+    expect(sortCountries([], 'name-asc')).toEqual([]);
+    expect(sortCountries([], 'population-desc')).toEqual([]);
+  });
+
+  it('does not modify the original array or the countries in it', () => {
+    const original = [sweden, norway, iceland];
+    const originalOrder = [...original];
+
+    const result = sortCountries(original, 'name-asc');
+
+    expect(original).toEqual(originalOrder);
+    expect(result).not.toBe(original);
+  });
+
+  it('recognizes valid and invalid sort option values', () => {
+    expect(isSortOption('name-asc')).toBe(true);
+    expect(isSortOption('population-desc')).toBe(true);
+    expect(isSortOption('area-desc')).toBe(false);
+    expect(isSortOption(null)).toBe(false);
+  });
+});