This chapter describes a formal approach to the modeling, specification, validation, and verification of the requirements, i.e., the required externally visible behavior, of a software system. The focus is on a special class of systems called critical systems, systems which require compelling evidence that the system delivers essential services and satisfies critical properties, such as safety and security properties. The objective of a formal approach to requirements is to produce a complete, consistent, and correct model of the requirements. The approach described relies on a formal specification of the requirements model in the SCR tabular notation, which allows several kinds of tool-based analyses of the model.