Among the National Science Foundation's (NSF) 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. IRFP aligns closely with the National Science Board's call for NSF to support the international science engagement of scientists and engineers. 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, which are to: (1) Introduce early career scientists and engineers to opportunities for international research collaboration; (2) Build research capacity and global perspective of participants; and (3) Forge long-term relationships between U.S. and foreign S&E researchers. This report presents the findings from the evaluation of 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. The report describes the methodology used to conduct the evaluation (Chapter 2); characteristics of IRFP applicants and their motivations for participation, and the host researchers' motivations for participating in the program (Chapter 3); participants' experiences in and perceptions of the program (Chapter 4); outcomes of the program (Chapter 5); and conclusions and implications of the findings of the evaluation. The evaluation demonstrated that individuals derive important benefits from the program, both on an individual and a collective basis. Former fellows and hosts strongly endorsed the IRFP program, and would recommend IRFP to their students and colleagues. IRFP offers an opportunity for interested researchers to develop their international research collaborations early in their careers. The following are appended: (1) NSF-Generated IRFP Highlights; (2) Non-Response Bias Study; (3) Detailed Description of Impact Analysis; (4) Detailed Description of Benchmarking Analysis; (5) Surveys; and (6) Comparison of IRFP Applicants to SDR.