This article reviews phase 2-3 clinical trial designs, including their genesis and the potential role of such designs in treatment evaluation. The paper begins with a discussion of the many scientific flaws in the conventional phase 2 --> phase 3 treatment evaluation process that motivate phase 2-3 designs. This is followed by descriptions of some particular phase 2-3 designs that have been proposed, including two-stage designs to evaluate one experimental treatment, a design that accommodates both frontline and salvage therapy in oncology, two-stage select-and-test designs that evaluate several experimental treatments, dose-ranging designs, and a seamless phase 2-3 design based on both early response-toxicity outcomes and later event times. A general conclusion is that, in many circumstances, a properly designed phase 2-3 trial utilizes resources much more efficiently and provides much more reliable inferences than conventional methods.