Because research synthesis enables one to determine either the overall effectiveness of a particular treatment or the relative effectiveness of different types of treatments, it is becoming increasingly popular as a tool in program evaluation. Numerous methodological problems arise, however, when research synthesis is applied to studies conducted in field settings. The present paper categorizes and discusses these problems as being threats to either the (1) internal validity (whether one can draw conclusions about cause and effect), (2) statistical conclusion validity (whether one's inferential statistics are capable of detecting cause-and-effect relationships), (3) construct validity (whether one's treatments and outcome measures are valid operationalizations of the independent and dependent variables of interest), or (4) external validity (whether one can generalize results to particular populations, settings, or time periods) of research synthesis (see Cook and Campbell, 1979). Specific recommendations are made for minimizing these threats to validity, in order to improve the quality of research synthesis in program evaluation. (Author)