The thesis aims to relate variation in life history and population variables to ecological factors. Data on small mammal were extracted from published reports and this information supplemented by measurements on specimens at the British museum (Natural History). Species are classified into ecological types accord- -1ng 'to habitat, diet, zonation, activity rhythm and nest site Latitude and breeding and growing season lengths of the habitat are also recorded. To avoid statistical difficulties, techniques removed the effects of both size and taxon. From comparisons of genera in different ecological classes, variation in life histories is closely related to diet: folivores, which have an abundant, low quality but highly predictable food, have shorter lifespans, are weaned earlier and reach maturity earlier than granivores and insectivores. When brain size is compared across the same ecological groups, fol1vores have relatively small brains. This may result from their less complex feeding niches. Developmental stages, such as the ages at ear and eye opening, vary only with nest site. Precocious young are associated with open terrestrial nests and altricial young with burrows. Increased predator pressure may select for faster development in exposed sites. Both home range size and population density are influenced by diet. Granivores and insectivores have relatively larger home ranges and 10Vier population densities. Fluctuations in density are greater for folivores and for populations living at high latitudes. Variation in body size is associated with diet, vegetation and sometimes climate. The calorific content of the food is negatively correlated with body size both within and across taxa. A set of hypotneses to explain variation in tae degree of sexual dimorphism in body size are reviewed. Some of these are tested and the results discussed in terms of the different selective pressures operating on different groups. Finally, two pervasive aspects of the analysis are discussed. The explanations for widespread differences between t:'xonomic groups are reviewed. The interrelation- -ships of diet, brain size, life histories and density fluctuations are discussed in the light of various explanations for their association.