A proper description of coalescence of viscous drops is challenging from an experimental, numerical, and theoretical point of view. Although the problem seems easy at first sight, consensus in the literature has still not been reached on how to predict a realistic coalescence rate given flow type, capillary number and viscosity ratio. Despite advances in algorithms and computational power, and the emergence of fully‐closed analytical results, a match between theory, experiment and simulation for drainage rates only appears in a severely limited number of cases. In this paper, several recent developments are reviewed, and a summary is made of several challenges that still lay ahead. magnified image