Introduction

This chapter provides a rationale for the study. It starts with a justification of the relevance of lexical knowledge in L2 learning, and, particularly within the context of Content and Language Integrated Learning (CLIL). Then, it identifies a research gap in CLIL lexical studies, which is related to the lack of studies exploring the quality of written input materials. The chapter proceeds to introduce the main variable under study, the notion of lexical complexity. Lexical complexity is understood as a multidimensional model comprising three constructs (Bui in Int Rev Appl Linguist Lang Teach 59:543–567, 2021; Bulté and Housen in Dimensions of L2 performance and proficiency: Complexity, accuracy and fluency in SLA. John Benjamins, Bui, 2012): lexical diversity, density and sophistication. The chapter explains how these constructs will be examined in the book and concludes with an outline of the structure of the book.