This chapter argues that the construction 'the idea is…' has undergone significant changes including (i) an increase in discourse frequency and a trend towards clausal complementation with 'to' or 'that', (ii) a decrease in morphosyntactic productivity and syntagmatic variability, and (iii) semantic developments towards grammatical meanings of intention, purpose or volition. In the spoken mode, 'the idea is…' also displays increased rates of intrusive /r/ and a tendency towards level or first-syllable stress in this grammatical use, which we interpret as phonological indicators of the dissociation from its lexical, nominal source. On the basis of data from different corpora (e.g. ICE-GB, BNC, ARCHER and the Time Magazine Corpus) we present synchronic and diachronic evidence showing that Hopper’s (1991) principles of grammaticalization – divergence, specialization, persistence and decategorialization – as well as Lehmann’s (1995) parameters of syntagmatic variability, bondedness and paradigmaticity can be applied to the variation and change found in this construction. We therefore conclude that the construction 'the idea is...' is undergoing grammaticalization.