Although most of the theoretical work in second language acquisition (SLA) over the years has led to advancement of theories of developing grammars, some recent SLA research has begun to investigate how those grammars are actually learned, relevant to current theories of learnability. This article (a) considers some of the claims for learnability principles that have been proposed within the first language (L1) context and the problems associated with their application to SLA, (b) examines four second language (L2) phenomena with respect to a promising variety of learnability theory labeled preemption, and (c) suggests in what ways research on learnability in SLA can contribute to the further development of learnability theory in general.