In this paper I explore the theoretical significance of phonologically conditioned gaps in word formation. The data support the original approach to gaps in Optimality Theory proposed by Prince & Smolensky (1993), which crucially involves MPARSE as a ranked and violable constraint. The alternative CONTROL model proposed by Orgun & Sprouse (1999) is found to be inadequate because of lost generalisations and technical flaws. It is shown that a careful distinction between various morphophonological effects (e.g. paradigm uniformity effects, phonological repair and ‘stem selection’) is necessary to shed light on the morphology–phonology interface. The data investigated here support affix-specific constraint rankings, but argue against any stratal organisation of morphology.