The extent to which form-focused instruction contributes to the acquisition of second language implicit knowledge is controversial. Whereas Krashen (1993) has argued that the effects of FFI on acquisition are peripheral, N. Ellis (this issue) sees FFI as facilitative and even necessary for developing implicit L2 knowledge. This article examines the role of FFI in developing implicit knowledge by reviewing 11 studies that have examined the effect of FFI on learners' free production. The review suggests that FFI can contribute to the acquisition of implicit knowledge and points to two variables that appear to influence its success—the choice of the target structure and the extent of the instruction. FFI involving extensive instruction directed at “simple” structures was more likely to succeed. However, limited instruction directed at complex structures also proved effective, provided that the target structures are readily available in noninstructional input.