Summary I. This review provides a background to those aspects of the biology and behaviour of Lepidoptera infesting stored products which may be important in terms of population dynamics. 2. The general biologies of four common species of Phycitid moth are described. 3. Both the fecundity and the fertility of the adult can be influenced by the diet of the female, the size of the female, the numbers of adults per unit area and a variety of environmental factors. 4. Egg mortality may be caused by sterility or cannibalism. Larval mortality may be caused by starvation or cannibalism. Crowding leads to emigration, reduced size and extended developmental period. Pupal and adult mortalities are of minor importance. Pupae may suffer developmental disorders and adults are killed by spiders and lost by emigration. 5. Parasitoids, predators and diseases, which also contribute to mortality of eggs, larvae, pupae and adults, are described. 6. The importance of planning and sampling in order to obtain detailed data on populations are discussed, together with the analysis of the data. 7. It is shown that Lepidoptera which infest stored products and the parasitoids associated with them have proved convenient experimental tools for the elucidation of certain aspects of population dynamics. A small number of studies have succeeded in identifying the main factors or processes responsible for fluctuation or regulation of the densities of these moths.