Like all natural sciences, behavior science has much to offer toward an understanding of the world. The extent to which the promise of behavior science is realized, though, depends upon the extent to which we keep what we know before us. This paper considers fundamental concepts in behavior science, including the concepts of behavior, stimulation, setting conditions, and language. In considering these concepts, we revisit comments from B. F. Skinner and J. R. Kantor and also consider some areas of behavior analytic research and the implications they have for reconsidering long-held assumptions about the analysis of behavior. We hope that, in considering our foundations, the vitality and strength of the discipline might be enhanced, our impact on science improved, and our future secured.Keywords Behavior . Behavior science . Stimulation . System building . Language All natural sciences have the potential to improve our understanding of the world. The natural science of behavior has the potential to improve our understanding of behavior, the subject matter of the discipline. The extent to which this potential is realized, though, depends upon several factors. These factors involve fundamental issues and have grave consequences for the entire discipline, the entire group of workers in behavior science, and the impact our discipline will make to the understanding of our part of the world, behavior. Indeed, much is at stake.Along these lines, we have previously addressed the value of becoming involved in interdisciplinary work and the problems associated with doing so in ways that diminish