Over the last fifty years, research into the development of second or other grammatical systems has been largely concerned with the stages through which language might evolve in the individual learner or in groups of learners. Explaining exactly how learner grammars undergo transition from one stage to the next has a much shorter history. However, there has in recent years been a rise in interest in the mechanisms that drive grammatical transition, particularly in online processing. Different theoretical approaches to the transition problem will be reviewed and exemplified here, including those in the few years that have elapsed since the first edition. Issues covered will also include (a) alternative definitions of grammar, (b) the acquisition of additional grammars after the second, (c) various other factors in the learner’s mind that help to shape the emergence of new grammatical structures. The chapter will close with an overview of the dominant questions still facing researchers embarking on investigations of grammatical transition.