In this paper we discuss various aspects of systematic or conventional polysemy and their formal treatment within an implemented constraint-based approach to linguistic representation. We distinguish between two classes of systematic polysemy: constructional polysemy, where a single sense assigned to a lexical entry is contextually specialized, and sense extension, which predictably relates two or more senses. Formally the first case is treated as instantiation of an underspecified lexical entry and the second by use of lexical rules. The problems of distinguishing between these two classes are discussed in detail. We illustrate how lexical rules can be used both to relate fully conventionalized senses and also applied productively to recognize novel usages and how this process can be controlled to account for semi-productivity by utilizing probabilities.