This review article focuses on second/foreign language (L2) learning motivation theories and studies. The author identifies the importance of L2 motivation and explains why there seems to be a lack of connection between research and pedagogy. Definitions of the concept and how it has been operationalized are discussed. The major part of this paper is a historical account of L2 motivation studies. First, the author traces back to the classical theory of L2 motivation, describes its background, development, and influence on subsequent research. Second, how the construct has gradually been modified and research agenda been reoriented through various scholars' studies and intensive dialogues in the 1990s are documented. Third, latest theories are discussed and more recent studies beyond 2000 are introduced from macro and micro research perspectives. Finally, Taiwan-based L2 motivation studies that have been published internationally are included as the final part of this literature review.