In terms of scope and level of abstraction, Lenz et al15 stated that “middle-range theories (are those) that are sufficiently specific to guide research and practice, yet sufficiently general to cross multiple clinical populations and to encompass similar phenomena.” In the quote from Lenz et al,15 the guidance for research and practice is much more direct than is that offered by grand theory; therefore, middle-range theory can be tested in the empirical world. The concepts or phenomena of interest can be coded objectively (by using either qualitative or quantitative methods) and it has the potential to postulate measurable relationships between the phenomena; thus it has a “time-relativistic distinction.”4 The generalizability of middle-range theory is further defined by boundaries that limit measurement of the person-environment interaction. Although testable across several different patient populations and environments, a particular middle-range theory does not address all patients in all environments. For example, Good and Moore's 14 theory on pain management applies only to adults who experience acute surgical pain and is appropriately tested only during the immediate postoperative period. Because of the aforementioned characteristics, middle-range theory is not as limited as grand theory in its typology and can be classified as either explanatory or predictive. A major role of middle-range theory is to define or refine the substantive content of nursing science and practice, and it should be an important focus of both nurse scholars and practitioners as we continue to build knowledge for the discipline.