Complex systems are naturally understood as combinations of their distinguishing characteristics or \definit{features}. Distinct features differentiate between variations of configurable systems and also identify the novelties of extensions. The implementation of a conceptual feature is often scattered throughout an artifact, forcing designers to understand the entire artifact in order to reason about the behavior of a single feature. It is particularly challenging to independently develop novel extensions to complex systems as a result. This dissertation shows how to modularly reason about the implementation of conceptual features in both the formalizations of programming languages and object-oriented software product lines. In both domains, modular verification of features can be leveraged to reason about the behavior of artifacts in which they are included: fully mechanized metatheory proofs for programming languages can be synthesized from independently developed proofs, and programs built from well-formed feature modules are guaranteed to be well-formed without needing to be typechecked. Modular reasoning about individual features can furthermore be used to efficiently reason about families of languages and programs which share a common set of features. / text