The large number of synthetic organic chemicals currently in production presents a major challenge for timely collection of detailed environmental data on each compound.  The concept of structure biodegradability relationships (SBR) has been applied to explain variability in persistence among organic chemicals in the environment.  Early attempts generally consisted of examining the degradation of a homologous series of structurally related compounds under identical conditions with a complex "universal" inoculum, typically derived from numerous sources.  This approach revealed that the nature and positions of substituents affected the apparent biodegradability of several chemical classes, with resulting general themes, such as halogens generally conferring persistence under aerobic conditions.  Subsequently, more quantitative approaches have been developed using principles of QSAR and often accounting for the role of sorption (bioavailability) in chemical fate.