This document reports the results of a meeting convened  by the Reference Group on Estimates, Modelling and Projections of the Joint United Nations Programme on HIV/AIDS (UNAIDS). The meeting brought experts from around the world together in Nairobi, Kenya in April 2009 to reach consensus on a standard definition of concurrent sexual partnerships, recommend methods for measuring concurrency in a population, and set out a future research agenda around the study of concurrent sexual partnerships and its association with HIV transmission.