This paper provides an overview of second language acquisition (SLA) research over the past several decades and highlights the ways in which it has retained its original applied and linguistic interests and enhanced them by addressing questions about acquisition processes. After discussing disciplinary contexts (SLA research and applied linguistics and SLA research and language acquisition studies), the paper focuses on traditions, trends, concerns, and controversies. It reviews research on the linguistic sequences of interlanguage development (morpheme accuracy order, verbal and phrasal negatives question formation, relativization, and word order) and research on SLA processes (language universals; linguistic universals; cognitive theories; cognitive processes, input, and interaction; and cognitive processes and evidence). The paper illustrates that SLA research has become increasingly bi-directional and multi-faceted in its applications, suggesting that these many applications to and from the study of SLA reflect the robustness and vitality of the field. (Contains 90 references.) (SM)