Human activities have greatly altered the natural geochemical cycles of several heavy metals, most notably lead derived from leaded-petrol and metal-smelting emissions. This inadvertent geochemical tracer experiment poses two challenges: understanding how anthropogenic lead affects human health and the environment, and quantifying its time-dependent distribution within terrestrial and marine systems. Accurate assessment of the latter relies on well-constrained historical and modern lead fluxes from proxy records and direct observations, lead source estimates from stable lead isotopes, and transport rate estimates from radionuclides. Numerous studies support the global-scale atmospheric lead fluxes principally derived from anthropogenic activities, the short lead residence time in the atmosphere and surface ocean, and the predominance of North American and European lead emissions. Emerging observations and models are currently addressing the time-dependent evolution of this reactive tracer in the atmosphere and oceans.