Incidental language acquisition is the language that is learned informally, outside the constraints of the typical classroom, and vocabulary is one of the key elements in language learning and knowledge. This unique text is the first comprehensive overview and hands-on methodological guide for researching second language (L2) incidental vocabulary acquisition. Expert contributors from around the world synthesize the state of the art by defining key concepts and laying out the major theoretical perspectives, research methodologies, empirical findings, and pedagogical considerations involved in incidental L2 vocabulary learning research. By connecting research techniques to the theory that underpins them, detailing practical steps for designing and conducting rigorous new studies, and highlighting areas that deserve additional research attention, they further set the agenda for future work in this field and put readers in a strong position to understand and carry out this research independently. This book will be an invaluable resource to advanced students and researchers of second language acquisition, vocabulary studies, applied linguistics, education, and related areas.