In this book, Peter Juul Nielsen examines the foundations of morphological theory from a structural-functional perspective on language as a sign system. He offers a framework for the analysis of morpheme relations based on a thorough discussion of syntagmatic and paradigmatic structure, indexical relations, zero as meaningful absence and morphological relations across grammatical categories. It is argued that when paradigmatically related morphological structures have different syntactic functions, the semantics of the paradigmatic opposition consists in the specification of functional potential. The framework is applied in three detailed studies of Danish nonfinite verbs presenting new accounts of their morphological structure, semantic coding and paradigmatic organisation.