This talk is about the nefarious molecules with which murderers and assassins have carried out their crimes, hoping always to escape detection. Yet many such deadly molecules have long been known, and many were once widely used by doctors to treat diseases. Murders committed with them posed problems for those who would bring the murderers to justice, and chemists were to provide essential evidence. More recently, secret agents assumed that newer deadly poisons could not be detected, like that used to kill Georgi Markov in London in 1978, Alexander Litvinenko in London in 2006, and Alexander Perepilichny in Weybridge in 2014. It was then up to forensic teams in the UK to discover what they had used.