Several field and laboratory studies of transport of colloidal and solute tracers in groundwater have observed an apparent increase in the mean velocity of colloids relative to non-reactive solutes. This phenomenon, referred to here as differential advection, is poorly understood and has been explained in terms of a variety of hypothesized processes acting at pore and larger scales. Only recently has there been an effort to develop quantitative models that are consistent with the hypothesized processes, and significant confusion and debate remains regarding the means of identification of, and the processes causing, differential advection. We present observations from a number of recent laboratory and field experiments and discuss their implications in the context of current conceptualizations and model representations of differential advection processes.