The simplicity of a pan-and-bag seepage meter makes it an attractive tool for a variety of hydrologic investigations, but the performance of this device is far from simple. Laboratory experiments show that the hydraulic head required to fill collection bags can range from 0.5 mm to several millimeters at flow rates typical of seepage meters, and this additional head will divert water away from the pan. A theoretical analysis gives a semianalytical expression for the flux captured by a seepage meter using a collection bag. The analysis shows that bag conductance, radius of the pan, and hydraulic conductivity of the stream bed can be combined to give a dimensionless term that characterizes seepage meter performance. Field experiments in a sandy streambed show that conventional seepage meters can resolve spatial differences in ground water flux but only when using a highly conductive collection system that is isolated from the current.