The authors present a theory that seeks to explain why parties change their political strategies, organizational characteristics and issue positions. Whereas most of the existing literature on party change deals with party systems, the focus here is on individual parties. Whereas much of the literature views parties as responding more or less gradually to socioeconomic change, change is here regarded as a discontinuous outcome of specific party decisions linked to party goals. This approach is placed in the literature by reviewing extant theories of party change. Our theory itself is initially advanced in a discursive section which suggests that change does not `just happen', but instead results from leadership change, a change of dominant faction within the party, and/or an external stimulus for change. The article then presents a more formal exposition of this theory, consisting of definitions, assumptions, and a series of testable propositions. It concludes with illustrative examples of this theoretical framework.