The unstable isotopes of chlorine have a brief history limited largely to production from sulphur isotopes. Recent improvements in accelerator targetry have made radiochlorine available from small cyclotrons, and concurrent research into labeling chemistry suggests both electrophilic and nucleophilic avenues for the synthesis of novel imaging probes. The prevalence of chlorine compounds from anthropogenic and natural sources offers varied and fertile ground for future scientific explanation with their radiolabeled counterparts.