David Block asks the question "What is second language acquisition (SLA)?" by offering an extended critique of one of the main theoretical approaches to second language acquisition: the "Input-Interaction-Output" model. He argues that a more interdisciplinary and socially informed approach to SLA research is in order, and he draws on recent work in sociolinguistics as well as SLA research influenced by sociolinguistic and sociohistorical approaches to language and language learning. In so doing, he demonstrates that there is a need to broaden the horizons of second language acquisition. Finally, he offers some important thoughts on the future direction of SLA research, and includes an extensive bibliography.