The term ecotoxicology is adopted in the sense of a comparative and integrated study of the undesirable effects of trace contaminants on the range of fauna and flora of an "ecosystem" or of a defined part or unit thereof. The importance of population changes over long periods of time is stressed. Sources, usage, and global trends of representative halogenated hydrocarbon (HHC) residues which appear as trace contaminants of environment, food, and living organisms are briefly compared: industrial solvents and intermediates, chlorofluoromethanes as an atmospheric pollutant, methyl bromide as a food and soil residue, HCH (hexachlorobenzene) as a fungicide residue, DDT, lindane (gamma-hexachlorocyclohexane), and dieldrin as insecticide residues, and polychlorinated biphenyls (PCBs) as industrial contaminants. A simple mathematical approach to the problem of relating inputs, persistence, and steady-state residue levels is explained. Persistence and biodegradation of representative compounds are compared. Attention is drawn to the persistence of hexachlorobenzene, the p,p'-dichlorodiphenyl- and hexachlorocyclopentadiene-derived moities of HHC residues. Ecotoxicological effects and their implications are discussed comparatively under the indirect effects of atmospheric pollutants and direct and indirect effects of trace contaminants of soil and aquatic ecosystems. Some conclusions related to research and "impact monitoring" are drawn.