Idioms constitute a subclass of multi-word units that exhibit strong collocational preferences and whose meanings are at least partially non-compositional. Drawing on English and German corpus data, we discuss a number of lexical, syntactic, morphological and semantic properties of Verb Phrase idioms that distinguish them from freely composed phrases. The classic view of idioms as “long words” admits of little or no variation of a canonical form. Fixedness is thought to reflect semantic non-compositionality: the non-availability of semantic interpretation for some or all idiom constituents and the impossibility to parse syntactically ill-formed idioms block regular grammatical operations. However, corpus data testify to a wide range of discourse-sensitive flexibility and variation, weakening the categorical distinction between idioms and freely composed phrases. We cite data indicating that idioms are subject to the same diachronic developments as simple lexemes. Finally, we give a brief overview of psycholinguistic research into the processing of idioms and attempts to determine their representation in the mental lexicon.