This thesis examines the range of variation regarding the pragmatic, semantic, morphosyntactic, phonological and lexical properties of incorporation constructions, focusing specifically on constraints on the variation. The thesis starts out with a review of the pragmatic, semantic and phonological variation in incorporation described in previous literature and the relevant constraints that can be formulated on the basis of earlier research. This review is followed by three in-depth studies of particular domains of morphosyntactic, pragmatic-semantic and lexical variation in incorporation. These domains of variation are analyzed based on data from a varied sample of languages. The morphosyntactic domain investigated concerns the possible morphosyntactic forms of incorporated elements. The pragmatic and semantic properties that are examined are the referentiality and modifiability of incorporated nouns respectively. With respect to the lexical properties of incorporation, the thesis addresses the question which types of verbs do and do not allow the incorporation of nouns. Finally, it is shown that the different theoretical accounts of incorporation proposed in the literature are only partly able to capture or predict the variation and constraints presented in this thesis. The thesis thus indicates that theoretical work on incorporation may generally benefit from paying more attention to the full range of variation in incorporation constructions and its constraints.