The concepts of the Common- and Same Type Rating are undoubtedly very attractive in today's cost-conscious airline operating environment. They allow the airlines to reduce their training and piloting costs while at the same time increasing scheduling flexibility by conducting mixed-fleet operations. However, certain types of flight deck differences beyond appropriate boundaries can result in increased workload and reduced crew performance. In addition, the introduction of advanced technologies, including those which enhance safety, can be delayed. This paper will attempt to shed some light on this complex issue by first defining what the Common/Same Type Rating is, by reviewing some of the experiences reported so far in mixed-fleet operations, and it will then discuss what some of the underlying human factors issues are and how known guidelines could be applied to future designs. The paper will conclude by trying to focus the industry's attention, (i.e., airplane manufacturers, airlines, and government agencies) on those areas in design, training, and scheduling where additional data is desperately needed.