Polychlorinated biphenyls (PCBs) persist in the UK environment despite restrictions on their use and disposal that have been in force for over a decade. Recent analytical advances have given us insights into the environmental behaviour of this complex group of chemicals and show that they may remain mobile in terrestrial and aquatic ecosystems for many years to come. Some PCBs are selectively degraded in the environment, while others resist degradation and continue to be recycled through living organisms. Simple models can be developed for the distribution of PCBs in the environment and are useful in predicting long term effects.