The last few decades have seen extensive research focusing of the relative effectiveness of different instructional options that can be employed in teaching grammar structures (e.g., deduction and induction, different types of corrective feedback, input-based vs. output-based practice). However, the contribution of such pedagogical intervention and the resulting knowledge of target language grammar are mediated by a number of factors related to a specific context, the properties of the features being taught and, most importantly, individual learner profiles. Nonetheless, research into the moderating role of individual difference variables has been scant, limited to only several factors, and seldom taking into account complex interactions between variables. The book seeks to fill this evident gap by investigating the mediating effect of selected cognitive and affective factors on explicit and implicit (or highly automatized) knowledge of the English passive voice. In doing so, the study sheds the so-much-needed light on the predictors of second language grammar knowledge but also, to some extent, on the usefulness of instructional techniques used to develop it.