Biology textbooks tend to assert the correctness of evolutionary concepts hut mention very little of the evidence that supports them. This gives the impression that evolutionary theory is poorly supported, which discourages acceptance of the theory. A case in point is the age of the Earth. Biology textbooks usually mention that the planet is ∼4.6 billion years old but neither tell how that was determined nor give evidence that the method (radiometric dating) is reliable. Students are therefore given insufficient reason to doubt that the Earth is any older than the 6000 years that the Genesis account suggests. Here, therefore, I review the evidence for an old Earth, to provide a concise but thorough reference for teachers who wish to supplement the meager information in textbooks with further details. Earth's age is relevant to biology because an old Earth is necessary for macroevolution to occur and also because some dating methods (e.g., dendrochronology, varves) use materials of biological origin.