It has been customary for linguists investigating the behaviour of idioms to focus primarily on their irregularities. The present work, which examines the idiom in the context of recent developments in syntactic theory, aims to uncover the regularities of idiom behaviour which underlie their superficial idiosyncrasies. The behaviour of Russian phrasal idioms offers a valuable testing ground for demonstrating the striking interconnections between the inflectional morphology of the Russian verb and idiomatic interpretation. Starting from the hypothesis that idioms are compositional with respect to the morphosyntactically driven features of their components, the analysis shows that the idioms under discussion reflect the feature values of their constituents transparently. The approach thus reveals a strong correlation between the semantic complexity and syntactic flexibility of idioms. The central hypothesis of this book, more technically, is that the degree of idiomatic frozenness can be defined in terms of X-bar projection levels and can be set out in terms of a ‘frozenness hierarchy’. The frozenness hierarchy is seen to be a hierarchy of specifications for a lexical entry on the assumption that lexical specifications of phrasal idioms cannot be overriden by syntactic specifications.