This chapter explains the resource concept in single-task performance. The resource concept is founded on the underlying assumption that the human operator has a limited capacity for processing resources that may be allocated to task performance. Two tasks demand more resources than one; therefore, timesharing can lead to a situation in which one or both have fewer resources than required, and hence, performance on one or both may deteriorate. The foundation for the resources concept in dual-task performance may be found in the early theoretical work of Broadbent (1958) and Kahneman (1973). The concept of resources in dual-task performance is most applicable and important in understanding and describing the effect of task difficulty. Different tasks are characterized by the dichotomy between stage-defined and code-defined resources. The enhancement of dual-task performance seems to result from circumstances in which a common mental set, processing routine, or timing mechanism can be activated in service of the two tasks.