This is a personal account of the investigation of DDE-induced eggshell thinning and the subsequent use of this information in the legal battle over the banning of DDT. The article focuses on the toxicological effects of DDT on the peregrine falcon,Falco peregrinus, although the effects on other species are briefly considered. The peregrine falcon population crashed throughout the Holarctic region in the 1950s and 1960s. Eggshell thinning was discovered in British peregrines in 1967 and was soon found to be a global phenomenon. The relationship between dichlorodiphenyldichloroethylene (DDE) residues and eggshell thinning was established by the beginning of the 1970s. Information on the effect of DDE on the peregrine falcon formed an important part of the evidence that led to the banning of DDT, and since a ban has been implemented in many countries the peregrine populations have improved in most areas.Key words: peregrine falcon, DDE, eggshell thinning.