We report a study on the deformation and breakup of drops in an impulsively started shear flow under Stokes flow conditions using boundary-integral simulations and video-microscopy experiments. Two independent techniques are used for determining the physical parameters of the system from the combined use of numerical simulations and experiments. Accurate breakup criteria (critical capillary numbers) are presented for a range of viscosity ratios. The time required for breakup events has a broad minimum corresponding to moderate shear rates. The size distribution of droplets produced by breakup events is shown to scale with the critical size drop for breakup in shear. A simplified model, based on this finding, is developed for the size distribution in a sheared emulsion. According to the model, the drop size distribution in a given emulsion depends only on the average initial drop size and the shear rate. (C) 2003 The Society of Rheology.