The syntactic and semantic properties of nonfinite verb categories can best be understood in relation to and distinction from the corresponding properties of finite verb categories. In order to explore these issues, it is necessary to provide a crosslinguistically valid characterization of finiteness. Finiteness is a prototypical notion, understood in relation to a language-specific finite verb prototype; nonfiniteness is therefore understood in terms of degrees of deviation from this prototype. The syntactic properties of nonfinite verb categories, so defined, can be considered from two perspectives: the functions of nonfinite clauses within superordinate clauses (e.g., argument and adjunct functions) and the internal structure of nonfinite verb phrases. Typical of the second aspect is that nonfinite phrases tend to be defective in one or another respect, relative to finite phrases, which may be understood in terms of lacking functional projections or features which are an obligatory part of finite phrases. This defectiveness relative to the finite prototype plays out also in the semantics; typically, certain aspects of the meaning of nonfinite phrases are not independently specified, but must be derived from semantic properties of a superordinate finite clause.