Man possesses a central system of limited capacity. Theorists at first described this system as a single limited capacity channel Two current theoretical alternatives to single-channel theory are (1) the undifferentiated capacity hypothesis that man possesses a pool of capacity units so that interference occurs oniy if the total number of capacity units that mental operations demand exceeds the system limit and (2) the hypothesis that some, but not all, mental operations require space in a limited capacity central mechanism and that any operation that requires space will interfere with any other operation that also demands space. Time on task fails as a sensitive measure of capacity demands because some task components require time but not full processing capacity. The secondary task technique uses the interference between a primary task and a secondary task to assess the extent to which the primary task makes processing demands on the central limited system. Processing demands have been measured for five categories of mental operation: (1) encoding, (2) multiple input, (3) rehearsal, (4) transformation, and (5) responding.