Allomorphy is a reaction of the morphological system to problems that the unrestrained application of inflectional and other rules creates at the phonological level. These problems are dealt with in some cases but left unattended in others. A diachronic analysis of English reveals that phonemically conditioned allomorphy originates from gradual sound change, with the old and the new variant forming a morphophonological paradigm. The historical stability as well as the synchronic motivation of allomorphy are claimed to be frequency-based. The higher the frequency, the longer the life expectancy. Synchronically, there is a (language- and domain-specific) frequency threshold above which morphophonological variation occurs and below which it fails to occur. The underlying logic of the model is that frequency encourages lexicalization at all linguistic levels. The relative ease with which high-frequency items are accessed enhances the tolerance towards formal variation, hence the emergence of natural phonological rules. As the application of these rules is context-dependent, allomorphy arises as a context-sensitive process. Repair strategies are also argued to be under the sway of frequency. Epenthesis is found in highly frequent structures while coalescence is reserved for less frequent ones. Frequency also determines the scope and the optionality of morphophonological rules. Phonemically governed allomorphy is shown to be a member of the larger family of variationist phenomena which are bound together by their sensitivity to frequency.