This study reviews the toxic effects of organophosphate (OP) pesticides on the immune systems and immune functions of invertebrates, fish, and higher vertebrate wildlife. The fundamental features and mechanisms of OP-induced immunotoxicity are illustrated with reference to parathion, chlorpyrifos, malathion, and diazinon. Immunotoxicity may be direct via inhibition of serine hydrolases or esterases in components of the immune system, through oxidative damage to immune organs, or by modulation of signal transduction pathways controlling immune functions. Indirect effects include modulation by the nervous system, or chronic effects of altered metabolism/nutrition on immune organs. Immunotoxicities are varied and include pathology of immune organs, and decreased humoral and/or cell mediated immunity. Altered non-specific immunity, decreased host resistance, hypersensitivity and autoimmunity are also features of immunotoxicity; although not all of these have been conclusively demonstrated in terms of pollutant exposure and immunotoxic effects in wildlife within individual experiments. Immunotoxicological biomarkers and biological monitoring tools are urgently needed to assess the extent of immunotoxicity in wildlife. Selection of universal biomarkers is hampered by the physiological diversity of immune systems in animals. However, by drawing on evidence from human epidemiology and tiered approaches in mammalian immunotoxicity evaluation, a selection of generic biomarkers of immunotoxicity in animals is suggested. Priorities for future research are also identified.