Why is diffusion in metals and on metal surfaces universal?

Recent experiments that determine mass diffusion D(s) on the close packed surfaces of vacuum compatible metals are reviewed. The results turn out to be approximately universal when scaled to homologous temperatures T/T(m), with T(m) the melting temperature. Similar behaviour for vacancy-driven diffusion in bulk metals has been recognized for decades. Remarkably, the uncertainty with which this scaling occurs is only ∼10%. Possible origins of the universality are discussed.