This essay by Truscott and Sharwood-Smith is a valiant attempt with a laudable goal. It seeks to show how different perspectives and disciplines can capture what is happening in acquisition and notably in L2 acquisition. Nonetheless, I think that the results are much closer to an elaborated grammatical theory than an elaborated processing theory (thus it seems less important to me than to the authors to choose one label over the other). Their essential, sensible idea is that where different grammars provide independent analyses of constructions, both are computed (even where the lexical items belong to only one grammar). Therefore conflict is experienced which produces computational demands. However this is pure grammar – just a more sophisticated situation where, as I like to argue, the L2 person has Multiple Grammars (Roeper, 1999), creating more computational work as well. In this respect, the situation is no different from L1 acquisition, where the course of acquisition involves generation and maintenance of multiple grammars, some of which are shed and some retained in the Final State.