Abstract: As Schütze (1996) and many others have shown, there are a variety of factors in addition to syntactic constraints that can affect participants’ responses in an acceptability judgment task. These include semantic, pragmatic, sociolinguistic, and prosodic constraints as well as general cognitive factors, task effects, and effects of language background and experience. Thus, a major challenge for syntax research is to be able to isolate the effects due to syntactic constraints and interpret their theoretical implications. Although a well-controlled factorial design is crucial, I argue that theoretical assumptions are equally important. In this presentation, I discuss three basic assumptions which are important for interpreting judgment data: (1) strict form-meaning isomorphism vs. flexible form-meaning mappings; (2) gradient vs. categorical notions of grammaticality; and (3) narrow vs. broad notions of grammatical competence. I situate these assumptions with respect to four families of theoretical frameworks: (1) derivational theories; (2) constraint-based theories; (3) competition-based theories; and (4) usage-based theories. I then discuss examples from our own studies of relative clause extraposition in English showing how different theoretical assumptions can lead to different interpretations of the same data. Finally, I show how additional data from corpus studies and elicited production tasks can shed further light on competing theoretical accounts.