Biological stoichiometry is the study of the balance of energy and multiple chemical elements in biological systems. A key idea in biological stoichiometry is the growth rate hypothesis, which states that variation in the carbon:nitrogen:phosphorus stoichiometry of living things is associated with growth rate because of the elevated demands for phosphorus-rich ribosomal RNA, a requirement for rapid growth. In this article, we synthesize studies in the cancer literature to test the growth rate hypothesis; consistent with its predictions, rapidly growing tumors have elevated ribosome content, key oncogenes are closely affiliated with regulation of ribosome biogenesis, and tumor development has physiological impacts on patient phosphate metabolism. We also describe a new eco-evolutionary model of tumor dynamics that incorporates stoichiometric mechanisms. Since biological stoichiometry is fundamental in its approach, other areas of biology in which multiple key resources are involved in generating important tradeoffs may also benefit from the new tools provided by stoichiometric theory.