A full color book written as a pocket guide (7x10 trim) and handy reference for advanced students and practitioners in molecular and cell biology, chemistry, and biophysics. It is based on the BioNumbers website, a peer-reviewed open-source database of numbers, created and curated by author Ron Milo, which receives nearly 40K unique hits per year. Well-illustrated and approximately 350 pages long, the book features several dozen question-driven vignettes spread over six chapters. They explore some of the key numbers for cell biology, focusing on quantities that help us to think about the sizes, concentrations, rates, energies, information content, and other numbers that describe the living world. Throughout the book, important calculations are explained and illustrated in greater depth.