Extensive reading is one of a range of activities that can be used in a language learning course. Ideally, the choice of activities to go into a course should be guided by principles which are well supported by research. Similarly, the way each of those activities is used should be guided by well-justified principles. In this article, we look at the principles justifying the inclusion of extensive reading in a course, and then look in detail at a set of principles guiding how extensive reading can best be carried out to result in substantial vocabulary learning. Extensive reading can result in a wide range of learning outcomes, but in this article we narrow our focus on vocabulary learning (for similar analyses of a wide range of vocabulary learning activities see Webb and Nation [in preparation]).