This report confirms anecdotal evidence that language teachers do find large classes to be problematic, collates data on class sizes in various parts of the world, and investigates the possibility of defining a large class, at least from the teachers'points of view. The first section is introductory. The second presents findings of a small-scale (n=46) investigation of the perceptions of native-speaking and non-native-speaking teachers of English as a Second Language (ESL) of the characteristics of large classes. The third section describes a larger study of ESL teachers' experiences with and perceptions of class size in Great Britain, Turkey, Indonesia, Madagascar, Mali, Japan, Greece, Jordan, and Spain. Section 4 discusses what the study tells about actual class sizes, and section 5 is a detailed analysis of the relationship between teachers' experiences and their perceptions of class size. It is concluded that class size varies across countries and probably across institution type. No evidence that teachers share a universal conception of ideal class size was found, but there appeared to be a positive correlation between the largest class regularly taught and both perceptions of an ideal class and perceptions of what a large class is. Further research is recommended. (MSE)