This chapter begin with a general definition of implicit and explicit CF and the classification of common CF strategies accordingly. However, emphasis is placed on the fact the individual strategies can vary in their degree of implicitness and explicitness and also in terms of whether they are input-providing or output-prompting. The significance of both cognitive-interactionist and sociocultural theories for predicting the effectiveness of implicit and explicit CF is considered next. The bulk of the chapter reviews the results of research, starting with meta-analyses of CF studies and then moving on to consider descriptive classroom-based studies and comparative experimental studies. Recasts (which can vary markedly in how implicit or explicit they are) have been found to be the most popular type of teacher feedback strategy but not in every context. Explicit CF results in higher levels of repair of errors. However, because of the numerous factors that affect the impact that CF has on acquisition (for example, whether or not noticing occurs, the grammatical target of the correction, and the intensity of the correction), it is not surprising that the results of the research are quite mixed and it is not possible to say which type of feedback is overall the more effective. The chapter concludes with some tentative generalizations and a plea for longitudinal studies of implicit/ explicit CF.