The articles in this collection discuss genetic engineering, nutritional amendments, the introduction of nonindigenous microorganisms, and other bioaugmentation techniques for enhancing biological activity to promote remediation of contaminated sites. Included are laboratory and field studies of the biological processes connected with in situ remediation of soil and water at sites contaminated with petroleum hydrocarbons, PAHs, PCP, TNT, insecticides, and chlorinated solvents. Site characteristics (e.g., soil porosity, nutrient availability) and methods for determining when bioaugmentation is needed are described. Among the bioaugmentation methods discussed are biodehalogenation systems, microbial filters, microbial ecapsulation, fertilization, niche adjustment, inoculation with laboratory-cultured and adapted microorganisms, and the use of mixed fungi and bacteria.