In the past two decades or so, washback or the influence of testing on teaching and learning has been intensely researched. Hypotheses and models have been formulated to uncover its nature, scope, patterns and mechanisms. Its intensity, length, specificity, intentionality and value have also been explored. Theorizing has done much to provide frameworks for empirical studies, which in turn have afforded evidence for or against concepts and theories of washback. An overview of the washback studies has shown that language tests have imposed both positive and negative washback on all aspects of teaching and learning, including teaching content, methods, materials. However, tests' influence on learning outcome remains largely nebulous due to scarcity of studies on this issue. Further, washback theories and models still leave considerable room for improvement before their explanatory power can be desirably enhanced.