The use of the incubated chicken egg as a test system to evaluate the effects of hazardous substances is reported. Several different methods to carry out toxicological as well as teratological studies are mentioned. Experimental toxicity testing and basic pathology studies on the extra-embryonal blood vessel systems of the chick embryo (yolk sac system and chorio-allantoic membrane) are described in detail. In this field, the application of the yolk sac system has some advantages compared with the chorio-allantoic membrane. Finally, the use of so-called "alternative methods" and the replacement of the Draize-test and LD50- studies are discussed.