Future data from galaxy redshift surveys, combined with high-resolutions maps of the cosmic microwave background, will enable measurements of the pairwise kinematic Sunyaev-Zel’dovich (kSZ) signal with unprecedented statistical significance. This signal probes the matter-velocity correlation function, scaled by the average optical depth (tau) of the galaxy clusters in the sample, and is thus of fundamental importance for cosmology. However, in order to translate pairwise kSZ measurements into cosmological constraints, external constraints on tau are necessary. In this talk I will present the current state of kSZ measurements, insights from simulations demonstrating the impact of astrophysical uncertainties, as well as recent progress in modeling and the tau profile of galaxy clusters.