A dual-bottleneck model for overlapping-task performance is proposed. A central bottleneck prevents central processes in overlapping tasks from operating in parallel. A response-initiation bottleneck prevents 2 responses from being initiated in close succession. Response requirements of the overlapping tasks were manipulated in order to demonstrate the existence of the latter bottleneck and to assess its role in delimiting overlapping-task performance. In several experiments, elimination of this bottleneck resulted in a strong reduction of underadditive interactions that serve as key indicators of parallel processing in overlapping-task performance. Results indicate that these interactions were caused primarily by the response initiation bottleneck instead of the central bottleneck and suggest that perceptual identification and categorization require the central bottleneck.