Pore-size distributions in filter media are determined by fluid-intrusion measurements. But many measurements indicate distributions that differ from what is expected.This paper reviews what sort of distributions are expected in a medium containing a random array of fibers or particles, or in a microporous membrane produced by the solvent-cast method. The paper then goes on to show examples where one type of fluid-intrusion measurement fails to show the expected random distribution, while the second one does, as long as certain conditions are met.