The literature concerning the occurrence and process chemistry of fission products in the nuclear fuel cycle is reviewed. A list of troublesome fission products, including ruthenium, zirconium, niobium and iodine is given, including a description of their chemistry in nitric acid solution and brief reasons for their troublesomeness. The major solvent extraction systems used to date in reprocessing, namely: ketones, ethers, amines and neutral organophosphorus compounds are described with emphasis on their fission product decontamination performance under highly active conditions.