Compost offers the potential to suppress root rots and vascular wilts caused by soilborne plant pathogens, as well as plant diseases affecting aerial plant parts. Many factors affect the degree of control obtained. They include the decomposition level (stability) of the compost, the types of microorganisms colonizing the organic matter after peak heating of the compost, plant nutrients released by the compost (fertility), its salinity, loading rates, and other factors. Biocontrol agents in composts induce suppression through various mechanisms, including competition, antibiosis, hyperparasitism, and the induction of systemic resistance in the plant (roots as well as foliage) to pathogens. Examples of each of the effects are reviewed.