The behavior of chemical species as they migrate through heterogeneous porous media is considered. The so-called "anomalous'' transport patterns frequently measured in these materials are quantified in the framework of a continuous time random walk (CTRW) formalism. The physical basis for application of the CTRW is discussed, and new solutions for the first passage time distribution are presented to covet the entire range of transport behaviors. Application of these solutions to analysis of experimental data is also discussed.