A key determinant in the success of computer-mediated communication systems (CMCS) and group support systems (GSS) is the task they are used for (Huber, 1984; DeSanctis & Gallupe, 1987). Task models and theories exist in the domain of non-mediated groups (e.g., McGrath, 1984; Wood, 1986) but application of these to GSS and CMCS has been spotty and the results equivocal (Zigurs & Buckland, 1998). Although research findings repeatedly suggest that the fit between task and computer-mediated communication technology is important, researchers have not yet been able to comprehensively describe or measure the dimensions of appropriate fit. This chapter describes the development and initial testing of an instrument to measure the perceived effectiveness of CMCS based on task type (hereafter PE measure). The PE measure extends prior research in several ways. First, it operationalizes the four major dimensions of McGrath’s task circumplex (McGrath, 1984; McGrath & Hollingshead, 1994), a model which frequently is used as a conceptual framework for studying GSS and CMCS (Dennis & Gallupe, 1993). Thus, it will be straightforward to integrate findings from studies that use the PE measure into the existing literature. Second, all four task types are incorporated into the PE measure, where prior research has focused primarily on generation tasks and, to a lesser extent, choice tasks. This comprehensive view of the overall task construct should benefit the process of theory-building as well as prediction in practical applications. Third, the PE measure has been tested successfully within heterogeneous task domains, suggesting that the instrument has validity and is relatively robust.