Three recent books take up different positions in the on-going debate about how, and out of what, to construct a theory of second language (L2) acquisition. Johnson (2004) advocates a "dialogically based approach", inspired by Vygotsky's sociocultural theory and Bakhtin's "dialogized heteroglossia", with which she would replace what she views as a prevailing "cognitive bias" in the field. Block (2003) similarly supports a "more interdisciplinary and socially informed orientation" to second language acquisition. But Block wants to reform rather than replace certain assumptions of what he represents as the best existing theory of second language acquisition, namely, Susan Gass' Input-Interaction-Output model (IIO model). Jordan (2004), on the other hand, argues forcefully that theorizing about second language acquisition must be based on a rationalist epistemology. He provides a set of "Guidelines" for theory construction, including six assumptions foundational to rationalist inquiry in general, and a five-point evaluation metric against which rival theories can be judged. He also passes on a list of six "practices to be avoided". Jordan encourages the cultivation of many, varied, theories so long as they observe the rationalist Guidelines. He goes on to criticize a broad sample of L2 research, commenting on whether specific proposals do or do not adhere to the Guidelines. This article reviews all three scholars' positions in this important debate, which has the potential to sharpen second language theorists' sense of what they are doing and how they should do it.