First awareness that direct topical application of xenobiotics to bird eggs could be harmful to avian development dates back to the turn of the century. The most widely documented evidence of embryotoxicity following direct exposure comes from petroleum contaminant studies, conducted with at least 10 different avian species. Many petroleum crude oils, refined oils, and waste oils are embryotoxic and moderately teratogenic to different species; LD50s are often less than 5 iL of oil per egg. Toxicity is generally dependent upon the PAH concentration and composition (presence of higher weight PAHs). Five of seven industrial effluents caused significant reduction of embryonic growth in mallards following brief immersion of the eggs. Of the insecticides, organophosphates have been the most widely studied with respect to potential for direct embryotoxicity and teratogenicity following spraying or immersion of eggs. Phenoxy herbicides including 2,4-D and 2,4,5-T have been the most widely studied class of herbicides with respect to potential embryotoxicity of spray application. However, more recent evaluations have indicated that this is not the most toxic class of herbicides. Paraquat was found to be highly toxic in at least three species. Herbicides with LC50s that occurred at ten times the field level of application or less for mallard embryos included bromoxynil with MCPA, methyldiclofop, paraquat, prometon, propanil, and trifluralin. Of different gaseous and particulate air pollutants, ozone and particulates rich in PAH content appeared to be potentially embryotoxic, based on laboratory studies. Environmental contaminants in all classes reviewed have been shown to cause physiological and biochemical disturbances in embryos or hatchlings indicative of contaminant exposure, organ damage, or delayed development. Residue studies have shown the presence of DDT, 2,4-D, 2,4,5-T, decamethrin, petroleum hydrocarbons, and methylmercury after direct exposure of eggs. Ability of xenobiotics to pass across the shell and its membranes as well as embryo uptake appear to be dependent both on the compound and the vehicle. Pesticides in aliphatic (nontoxic oil) vehicle were generally more toxic than ones in aqueous emulsion due to better penetration. Field studies have documented the embryotoxicity of petroleum following transfer from plumage of adult birds to their eggs. Few attempts to measure effects of spraying pesticides near bird nests have been documented. However, application of pesticide mixtures including organophosphate insecticides and fungicides to orchards resulted in embryo ic mortality in mourning dove nests. Future studies are needed to focus on field exposures in multiple species. Comparative laboratory studies are needed taking into consideration shell thickness and porosity to determine whether species such as passerines may be more sensitive. Additive and possibly synergistic effects may occur where xenobiotics may be only slightly to moderately toxic alone. Therefore, further studies examining the effects of pesticides routinely applied in combinations of two or more are needed, as are air pollution studies examining multiple contaminants and species.