The value of long-term biological studies of single spills, repeated small spills and chronic sources of pollution has long been recognized, but relatively few examples are to be found in the scientific literature. A small number of case histories of long-term studies are described here that illustrate a range of biological effects resulting from chronic inputs in the form of repeated small spillages, refinery effluents and discharges from offshore platforms. These studies have taken place around refinery effluents and spill sites in Milford Haven between 1960 and the present, around a refinery effluent in Southampton Water from 1969 to the present and in two oilfields in the North Sea since 1973 and 1975 respectively. Observed effects of chronic discharges in coastal waters range from localized and subtle to severe and long-lasting. In the producing offshore oilfields investigated so far, widespread severe effects have not yet been observed and such fields may present relatively few long-term biological problems. An important gap in knowledge that deserves attention is the biological effect in those fields where oil-based drilling muds have been used extensively, as preliminary evidence suggests that substantial quantities of oil can reach the seabed from this source. While it is recognized that the total impact of any pollutant on marine systems may be impossible to measure, the factors that determine the form and size of biological effects may be induced from the above examples and other examples from the scientific literature. A summary of these factors is presented. It is concluded that the severity of effects depends largely on the type, nature and duration of the pollutant, the physical nature of the receiving environment, and its biological nature and sensitivity. Where pollutant loads are reduced or removed (naturally or by pollution control measures), biological damage is usually followed by recovery of the system to an apparently healthy condition. Subtle effects may, however, persist for considerable periods of time.