Success (rate of acquisition and ultimate attainment) in second language acquisition varies greatly. One explanation of this variability lies with learners' individual differences (ID). "Many of us believe that individual differences that are inherent in the leaner can predict success or failure in language learning" (Lightbown and Spada, 2013, p. 5). The present synthetic review focuses on the effects of individual differences, but more narrowly focuses on success in input processing (IP), and specifically, processing morphosyntax in processing instruction (PI) and structured input (SI) research. Individual differences address the following questions: Do individual differences mediate the effects of a pedagogical intervention, processing instruction? What are the characteristics of learners who benefit the most from processing instruction? or only somewhat? or the least or even not at all?