This book highlights the importance of individual learner differences in learning English as a foreign language and reports the findings of a study which investigated the impact of two personality traits, which are, openness to experience and ambiguity tolerance, on target language attainment among Polish secondary school students. The book provides an exhaustive overview of the theoretical issues and existing research related to personality, emphasizing the two traits under investigation, openness, and ambiguity tolerance, which are the focus of the empirical study reported later in the book. The empirical investigation explored relationships between openness to experience and ambiguity tolerance, as well as their impact on attainment in learning English as a foreign language. Moreover, it also aimed to shed light on the link between these traits and students' assessments (i.e., self-assessment and school grades). The findings of the study provide a basis for proposing specific profiles of foreign language learners with different levels of openness and ambiguity tolerance.