Bartelt, N. C.Goldberg, J. L.Einstein, Theodore L.Williams, Ellen D.Heyraud, J. C.Metois, J. J.Step motion on surfaces can now be measured quantitatively. We present a formalism for analyzing equilibrium step fluctuations and apply it to real-time reflection electron microscope observations of step motion on Si(111). The time correlation functions of the step positions and of their Fourier components are compared with predictions from Langevin equations for two extreme mechanisms for step motion: edge diffusion and terrace exchange.Article