This article considers the interaction of phonotactics and diachrony. I argue two things: (i) language-specific phonotactic constraints on phonological forms can inhibit otherwise regular innovations, and (ii) the fact that such phonotactically-motivated process inhibition occurs in historical phonology is itself evidence for the reality of phonotactic constraints. I assume that there is a difference between those gaps in a language’s lexicon which are due to chance (‘A-gaps’) and those which are ruled out by the grammar (‘S-gaps’) and I consider some evidence in favour of this view. I consider two case studies where an understanding of phonotactics is necessary to analyse the patterning of change: Mid-Scots θ-debuccalisation and a late Middle English syncope. I ground the discussion in arguments about what phonotactic constraints are, and how they can be involved in diachrony. This involves a consideration of a number of examples from English, including onset-OCP-related constraints, the OCP (sibilance) constraint, and the constraint which imposes the defective distribution of h.