This book addresses the L2 acquisition of the pro-drop parameter as one of the oft-cited parameters in Chomsky’s Principles and Parameters (P&P) Theory. Studies in the literature claim that triggering one of the properties of the pro-drop parameter leads to the acquisition of the other properties which in turn results in resetting the parameter. The book explores whether focusing on one of the properties of the pro-drop parameter through inputting different types of positive/negative evidence could trigger the L2 acquisition of (the other properties of) the parameter. It offers a useful review of issues such as L2 learners’ access to UG in resetting the pro-drop parameter, markedness and the pro-drop parameter, the type of evidence needed to reset the pro-drop parameter, resetting the pro-drop parameter, and the clustered properties of the pro-drop parameter. Employing an empirical approach, the book challenges the resettability of the pro-drop parameter and reports on the theoretical, empirical, and pedagogical implications of the findings. The intended readership of the book is postgraduate students, instructors, and researchers involved in second language acquisition research.