Fentanyls are solid analgesic–anaesthetics used in human and veterinary medicine. The science relevant to their use in defence and security is presented in this chapter. This includes a timeline of the pharmaceutical development of fentanyls and their medicinal uses. Their rise to prominence as incapacitating agents occurred when the Russian Special Forces used a mixture of two fentanyls, carfentanil and remifentanil, to facilitate resolution of the Moscow Theatre Siege in 2002. The identification of the composition of the mixture, enabled by analysis of clothing and a urine sample from the casualties, is described. A resurgence of apparent military interest in fentanyls by some Governments underpins the importance of developing a better awareness of the dangers posed by these toxic chemicals. This chapter discusses how research about the biological effects and chemistry of fentanyls serves as an information source to support the Chemical Weapons Convention (CWC).