Porous systems play a significant role in controlled release. Porous membranes and matrices can be used to store drug prior to release, and pore structure plays a significant role in determining release kinetic profiles. In this chapter we review methods used to measure pore structure and mathematical models used to relate pore structure to drug transport properties. Steric and hydrodynamic interactions between drug and pore walls, pore tortuosity, and variation in pore width are identified as factors affecting transport. Percolation theory, which addresses connectedness of random pore networks and its effect on overall releasability of drug and release rate, is discussed. Concepts developed for porous systems can be applied, with some modifications, to transport in other heterogeneous systems, such as tissue interstitium and hydrogels.