Chemical oxidants are increasingly being used for the in situ destruction of organic contaminants in groundwater. The most common implementation involves using an injection/withdrawal system to circulate oxidants (e.g., potassium permanganate, hydrogen peroxide, and Fenton's reagent) through a source zone containing a dense non-aqueous phase liquid (DNAPL). Because the efficiency of chemical oxidation is highly dependent on geological heterogeneities, effective delivery schemes are essential for successful remediation. This article reviews the impact of heterogeneities on the success of in situ chemical oxidation. Physical heterogeneities are primarily concerned with the permeable pathways along which oxidants are transported to the zone of contamination. Chemical heterogeneities refer generally to variability in geochemical properties that also bear on the efficiency of oxidant flooding. Both types of heterogeneities work against bringing the oxidant to zones of high contaminant saturations. The highly heterogeneous distribution of contaminants and difficulties in characterization make it difficult to target specific zones for treatment. As a result, large volumes of sediments could be treated whether they are contaminated or not. Heterogeneities in hydraulic conductivity at most sites provide an intensive dose of chemical reagents along permeable pathways and little treatment of low-conductivity zones. Large quantities of oxidizable materials in geologic units are capable of consuming the oxidant during delivery. Reaction products [e.g., CO2, MnO2, and Fe(OH)3] tend to plug the porous medium, especially in zones with large contaminant saturations. The oxidant flood is diverted away from these zones, making the flooding inefficient.