Over a decade ago, the National Science Board (NSB) highlighted the importance of international collaboration in its call for increased government commitment to promoting international science and engineering (S&E) research and education. The NSB also identified the National Science Foundation (NSF) as having an important leadership role in international S&E research and education activities. Among NSF's postdoctoral programs, the International Research Fellowship Program (IRFP) is unique in its emphasis on providing postdoctoral fellows with international research experiences. Established in 1992, IRFP provides financial support to postdoctoral scientists for a research experience abroad lasting from 9 to 24 months; there is no restriction on the geographical location of the hosting institution. NSF contracted with Abt Associates to conduct an evaluation of IRFP, to gather evidence about whether the IRFP program has achieved its goal of furthering the collaborative activities and international partnerships of early career STEM researchers. The evaluation found evidence that the IRFP program is meeting its goals. This report summarizes the findings from the evaluation of the International Research Fellowship Program (IRFP), which was designed to investigate the characteristics of IRFP applicants and their motivations for participation, the host researchers' motivations for participating in the program, participants' experiences in and perceptions of the program, and outcomes of the program. Findings from the evaluation of IRFP revealed important benefits of the program, on both an individual and a collective basis.