How do you implement effective testing strategies for custom software?

Testing strategy follows the test pyramid: many unit tests (fast, isolated, testing individual functions), fewer integration tests (testing component interactions and database queries), and minimal end-to-end tests (testing complete user flows). Unit tests: test business logic in isolation using mocks for external dependencies, aim for 80%+ coverage on core domain logic, and use property-based testing (fast-check, hypothesis) for complex algorithms. Integration tests: test API endpoints with a real database (use test containers or in-memory databases), test external service integrations with contract tests (Pact), and test database migrations on realistic data. End-to-end tests: use Playwright or Cypress for critical user flows, limit to 10-20 core scenarios (login, purchase, key features), and run in CI against a staging environment. Additional testing types: performance testing (k6, Gatling) to establish baselines and detect regressions, security testing (OWASP ZAP for automated scanning, manual penetration testing for releases), accessibility testing (axe-core integrated into CI, manual screen reader testing), and visual regression testing (Percy, Chromatic for UI changes). Test data management: use factories (factoryboy, fishery) for test data generation, implement database seeding for integration tests, and maintain anonymized production data copies for realistic testing. CI integration: run unit tests on every commit (under 5 minutes), integration tests on PR (under 15 minutes), and full E2E suite on merge to main (under 30 minutes).