Multiple resource theory and related principles hold that resources are differentiated both within and between cerebral hemispheres (C. D. Wickens, 1984, 1991, 1992). An opposing view is that each hemisphere constitutes a pool of undifferentiated resources (A. Friedman, M. C. Polson, C. G. Dafoe, & S. J. Gaskill, 1982; M. C. Polson & A. Friedman, 1988). Here the authors compare the theories by using dual-task methodology, drawing on tasks emerging from factor analytic studies of lateralized processes. Selective interference occurs when 2 tasks draw on the same process in the same hemisphere but not otherwise, a conclusion further supported by analyses of difficulty trade-off. Differentiation indeed exists within and between hemispheres, but evidence is found for resources not envisaged in multiple resource theory. The results suggest that orthogonal processes represent orthogonal resources, and a full accounting of them is likely to be needed in any complete multiple resource model.