Introduction

Educated native speakers of English know 15,000 to 20,000 word families ( Goulden, Nation and Read, 1990 ) and they learn most of these words incidentally through reading and listening. It is not clear to what extent L2 words are learned incidentally. However, the contribution of incidental vocabulary learning to L1 lexical growth highlights its potential value for L2 learning. There are many different lines of research investigating incidental vocabulary learning and this volume includes several key studies from five areas that continue to generate interest. The first two areas of interest are closely related: the extent to which words are learned incidentally through reading, as well as incidental vocabulary learning through listening, and watching television. The former has received the majority of the attention, while the latter has great potential for further research over the next decade. The third area of research looks at the different factors that influence incidental vocabulary learning. The fourth and fifth lines of research are linked by their use of corpora in their designs. Corpus-driven studies of vocabulary have indicated the importance of vocabulary for understanding spoken and written text, as well as the potential for incidental learning through reading and listening to different amounts of language. These lines of research are important because if we cannot understand speech or writing then we are unlikely to learn words through listening and reading. Similarly, if we hope to have significant growth through reading and listening, then it is important that we have some idea of how much listening and reading are required for learning to occur. These corpus-driven studies have been of much interest in recent years. Moreover, a recent study by Schmitt, Cobb, Horst and Schmitt (2015 ) reveals the need for further research in this area.