This chapter reviews research on whether individual differences in psychometric intelligence, working memory, and other less investigated variables, such as emotion and personality, affect implicit learning, with particular focus on Reber's evolutionary theory and Kaufman's dual-process theory for implicit learning. The review shows that while the null effects of psychometric intelligence on implicit learning seems robust as both theories claim, those of working memory were unclear due to methodological insufficiency. For the effects of emotion and personality, further investigation is needed as studies in this direction have just begun to proliferate. The chapter concludes that the research findings on the effects of these individual difference variables on implicit learning are still inconclusive, except for psychometric intelligence, and provides suggestions for future research.