Temporal media is information that is directly consumed by a user, and that varies with time. Examples include music, digital sound files, computer animations, and video clips. In this paper we present a polymorphic data type that captures a broad range of temporal media. We study its syntactic, temporal, and semantic properties, leading to an algebraic theory of polymorphic temporal media that is valid for underlying media types that satisfy specific constraints. The key technical result is an axiomatic semantics for polymorphic temporal media that is shown to be both sound and complete. Categories and Subject Descriptors D.G.1 [Programming Languages]: Formal Definitions and Theory General Terms Media, multimedia, polymorphism, temporal, algebra, formal semantics, axiomatic semantics, soundness and completeness, functional programming.