This proposal describes a series of experiments aimed at addressing the questions of the stability of crystal surfaces at elevated temperatures. The relationship of three types of instabilities on the relatively open surfaces of metals are particularly interesting; surface roughening, surface melting, and irreversible surface reconstructions. Metal surfaces are exceptional candidates to study all three instabilities since the temperatures for these different structural transformations (whether already measured or theoretically predicted) are all very similar for this face. All of these studies require detailed information on the temperature dependence of the long range correlations. This information will be obtained through accurate measurements of low energy electron diffraction profiles. Understanding roughening on metal surfaces will be of extreme interest since the high concentration of active adsorption sites present above the roughening temperature are critical to many important catalytic processes. In addition, surface roughening and surface melting are now recognized as important factors in the growth of semiconductor superlattices and metal-semiconductor interfaces.