Very little in our human experience is truly comparable to the immensely crowded and bustling interior of a cell. Biological numeracy provides a new kind of understanding of the cellular world. This book brings together up-to-date quantitative data from the vast biological literature and uses the powerful tool of "back of the envelope" estimates to reveal fresh perspectives and insights from numbers commonly encountered in cell biology. Readers gain a feeling for the sizes, concentrations, energies, and rates that characterize the lives of cells-- thereby shedding new light on the microscopic realm.