This chapter considers interactions and behaviour as dependent variables, with the context or classroom environment of the focus of attention as the independent variable. It reviews some of the ways in which classroom contexts have been conceived and researched. The chapter focuses on class level contexts, in terms of the physical features of classrooms, the number of pupils in a class, the allocation of pupils to classes on the basis of ability, the interactive and social characteristics of classrooms, and the classroom psychological environment. It highlights the research on class-size differences and within-class grouping practices that took place at the Institute of Education, United Kingdom. The chapter also focuses on two main types of research: research on the effects of class size on pupil's academic performance and, research that investigates effects on classroom processes such as teaching and pupil attention. Streaming involves allocating students to classes for most or all curriculum subjects based on general assessments or combined attainment results.