This book fills some of the major theoretical gaps for the popular teaching form of Content and Language Integrated Learning (CLIL). It re-examines the basis for CLIL from a cognitive perspective and gives an overview of the foundations of problem solving, thinking and language processing. Besides that, it presents a study in which the language and content-focused cognitive processes of learners during task solving are investigated. The data show that the use of an L2 has an impact on conceptual thought in that it can enhance semantic processing.