Total animal biomass depends on four factors: (1) food supply, (2) the efficiency with which animals consume available food, (3) the efficiency with which animals convert consumed food into biomass, and (4) the rate at which animals lose biomass to the environment through respiration or death. Each of these factors may change through geological time because each is a function of animal ecology and physiology. Animal ecology and physiology, in turn, are products of interacting evolutionary and environmental factors. The direction of change in animal biomass through time may be predicted given knowledge of environmental and ecological change. At a finer level, physiological differences among phyla or other higher taxa suggest that they would have had differential responses to specific environmental changes. Physiological features shared by all of life, such as the dependence of metabolic rate on ambient temperature, suggest that even a coarse time-series of relative changes in animal biomass may enrich understanding of biogeochemical cycling among all organisms, including phytoplankton and microbes. Changes in the abundance of skeletal material in shallow marine deposits through geological time indicate that the biomass of benthic skeletal invertebrates has fluctuated significantly on timescales from millions to hundreds of millions of years. During the Ordovician radiation, increase in the complexity of animal food webs and increase in the efficiency of animal communities in removing available food from the water column and sediment appear most likely to account for a secular increase in animal biomass. Decrease in animal biomass after the end-Permian extinction appears to have been driven by a combination of factors but particularly decreased aggregate growth efficiency and consumption efficiency. Comparing biomass and diversity trends through other major transitions in the history of animal life has the potential to shed light on the relationship between physical environmental change and ecosystems evolution.