Conceptually and theoretically, it is easy to see how short gaps and concurrent partnerships fuel the spread of sexually transmitted infections (STI) and HIV in populations. The consequences of concurrency are bounded by the duration of the infectious period and by the relationships between concurrent partnerships and other dimensions of sexual behavior. Consequently, it is difficult to predict how concurrent partnerships or short gaps may be related to the spread of STI and HIV in a particular epidemiologic context in empirical reality. In recent years, analyses of sexual behavior data collected through nationally representative surveys revealed similar prevalences of concurrent sexual partnerships in Western populations. The association between concurrent sexual partnerships and STI/HIV risk is complicated. At the population level, recent findings suggest that the ecological association between polygyny and HIV prevalence is negative at the country level and at the sub-national level. To address the need for agreed-upon standard definitions and measures of concurrent sexual partnerships, which will facilitate comparisons across time and settings, the Joint United Nations Program on HIV/AIDS (UNAIDS) Reference Group on Estimates, Modelling, and Projections convened a meeting in April 2009. The recommendations developed at this meeting include suggestions for a definition, indicators, and measures of concurrency.