Biodegradation is the dominant pathway for the environmental transformation of most chemicals and information on a chemical's biodegradability is essential for proper risk assessment. But there are few methods for predicting whether or not a chemical is biodegradable, since this depends on the chemical's structure as well as on the environmental conditions that it encounters. This text deals with quantitative structure-biodegradability relationship models (QSBRs), emphasizing the biological and ecological part of the biodegradation process. Surveys are given of the microbial aspects of biodegradation and the methods available for testing biodegradability. Recent trends and methods in biodegradation modelling are reviewed, including contributions on computerized biodegradability prediction systems. Some of the recently developed models for assessing risk and ecological impact in aquatic and terrestrial environments have been validated, and this process is discussed.