Research on the role of the first language (L1) in second language (L2) learning is reviewed, offering historical background but focusing primarily on work within the last two decades. Attention is given mainly to two aspects of the L1-L2 relationship: positive transfer of knowledge from L1 in the process of learning L2, and negative transfer, or interference. Related theories are traced from the 1960s, and empirical research from the 1970s. Research topics examined include L2 learner errors either traceable to or not traceable to L1, L2 development independent of L1, production and reception errors, and transfer of properties shared by L1 and L2, including language universals. More recent research on specific constructions promoting or inhibiting transfer, on learner perceptions of L1 and L2 in transfer, and on transfer in specific languages is highlighted. It is concluded that in view of mixed research results and the limitations of existing knowledge, it is best to view L1 as a contributing factor in L2 development. (Contains 62 references.) (MSE)