Yeast and animals cells control cell growth and division in different ways. In yeast, rates of cell growth are strictly controlled by nutrient availability. In nutrient-rich environments, growth rates are high and cells are large. In contrast, when nutrients are limiting, growth rates are slower and cells are smaller. Cell-size checkpoints function to ensure that yeast cells divide only at a critical size dictated by nutrient conditions; they therefore ensure that proliferation rates in yeast are appropriately tailored to environmental conditions. In animal development, cells are under the influence of a variety of extracellular stimuli such as nutrients, growth factors, mitogens and various patterning inputs, examples of which are shown. These signals mediate cell-to-cell communication and act to control both cell size and cell numbers, in order to ensure correct organ and organismal growth. Under these circumstances, strict cell-size checkpoints may not be necessary. Rather, overall proliferation is probably regulated by the independent but coordinated control of growth and division by diverse stimuli.