The simultaneous presence in a learner's grammar of two features that should be mutually exclusive (optionality) typifies second language acquisition. But generative approaches have no good means of accommodating the phenomenon. The paper proposes one approach, based on Truscott and Sharwood Smith's (2004) MOGUL framework. In this framework, development involves items being written in the lexicon during processing, in accordance with UG principles embodied in the processors, and their resting activation levels gradually rising as a consequence of repeated use. Stored items compete with one another for inclusion in the representations constructed during processing, with success based on resting levels plus demands of the current processing task. Optionality occurs when two rival items coexist and neither has yet achieved a resting level at which it consistently triumphs over the other. The account is applied to various cases of optionality in second language acquisition. (Contains 1 figure and 4 notes.)