Those who are involved in research in gaming and simulation find themselves in a dual position. One reason for this duality relates to the fact that researchers in gaming and simulation represent two distinct branches of science: the design and analytical sciences. The basic idea of the design sciences is to build and assess artifacts. The scientific method of the analytical sciences aims at developing and testing theories. Both pay attention to different notions of causality, and apply different criteria of success. This article presents a framework both for artifact assessment and theory testing, which helps to better understand the distinct scientific and professional issues involved, and to offer directions for cross-fertilization.