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A modular monolith is a single-deploy app intentionally partitioned into domain-aligned modules with clear interfaces to get monolith simplicity plus service-like boundaries. Core patterns: module-per-domain (bounded contexts) owning models and persistence; ports-and-adapters (hexagonal) to isolate domain logic from infra; plugin/extension modules for extensibility; and data-ownership with domain events and eventual consistency for cross-module workflows. Real-world examples—Shopify, GitLab, Basecamp—show modularize-first then extract selectively. Best practices: enforce boundaries via reviews, linters and packaging rules; prefer synchronous calls inside modules and asynchronous events between; keep single source of truth per entity; automate integration tests; monitor coupling; refactor incrementally—start by modularizing a high-change area.