First, this paper will briefly discuss the importance of human workload in aviation systems. The purpose is to provide the motivation for the following section on defining and measuring workload. Because different measurement ways do not always agree with each others, we will relate workload to the psychological concept of attention and suggest how theories of attention can help solve the problems. Next, we will list some of empirical workload studies using a varieties of measurement techniques. Finally, this paper will end up with a brief discussion of prospects for workload research.