In the last fifteen years, there have been considerable changes in and additions to our understanding of chemisorption. Before this time, surfaces were usually pictured as homogeneous, and it was generally believed that one binding energy characterized adsorption at a particular coverage and that the heat of adsorption varied strongly with coverage. While desorption processes usually were found to obey Arrhenius expressions, pre-exponential factors showed wide and unpredictable variations. Similarly condensation kinetics could only be described in vague terms as occurring through either site occupation or precursor-controlled kinetics, but there was no concensus as to the details of the energy transfer process or the potential surfaces involved.