This chapter discusses the individual differences that have been shown to influence the incidental acquisition of vocabulary through reading. The chapter introduces readers to the concept of individual differences and then provides them with a literature review investigating the variables age, gender, second language (L2) proficiency, L2 reading proficiency, L2 vocabulary size, motivation, freedom of choice, interest in topic, reading purpose, topic familiarity, background knowledge, cognitive factors, and vocabulary-learning strategies. This review focuses on variables that are yet to be adequately examined. The chapter ends with a reflection on the aspects that require further research.