Modular monadic semantics is a high-level and modular form of denotational semantics. It is capable of capturing individual programming language features and their interactions. This thesis explores the theory and applications of modular monadic semantics, including: building blocks for individual programming features, equational reasoning with laws and axioms, modular proofs, program transformation, modular interpreters, and compiler construction. We will demonstrate that the modular monadic semantics framework makes programming languages easy to specify, reason about, and implement.