It is difficult to evaluate public programs of any kind but the task is particularly complex when the programs are intergovernmental in nature. Such programs often necessitate a long developmental period before they are likely to have measurable impact. The early stages are often devoted to developing support from local power structures. These programs have many unintended consequences—both positive and negative—that short-term evaluations will usually fail to note. An exploratory study of the Federal Health Planning and Community Mental Health programs is used to illustrate the importance of the “time dimension” in evaluation research.