Land-development scenarios as a means of representing the future have been in the planner's toolkit for several decades. In this paper we provide a systematic view of four basic issues that concern scenarists and scenario users—the concepts, functions, credentials, and efficacy of land-development scenarios. Drawing upon the wealthy and expanding pool of knowledge and experience as reported in the literature, we put forward the notion that a land-development scenario set is both a bridge that connects the process of modeling with that of planning and a cognitive apparatus that stretches people's thinking and broadens their views in planning. The dual function entitles a scenario set to be a favored member of a family of innate instruments that humans operate in making decisions. Under this overarching framework, we propose three credentials that are by no means exhaustive yet are claimed to be essential for a scenario set to perform best the dual function. These are plausible unexpectedness, informational vividness, and cognitively ergonomic design. After exploring the efficacy issue of a scenario set with respect to its impacts on communities at large, we suggest that basic research efforts be underway that aim at the development of unified theories of land-development scenarios, or even scenarios in general, under a possible name of scenariology—the study of scenarios.