The 137Cs method uses traces of nuclear activity to measure soil loss, and constitutes a relatively swift and cost-effective means of collecting data on soil erosion as compared to more conventional methods. This guide covers the main principles, sampling strategy, gamma spectroscopic measurements, estimation of erosion rates with conversion models, data interpretation and validation of erosion models. The Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture of the IAEA has been working on this method for over 20 years.