A classical thermodynamic description of a surface requires the introduction of a number of energetic parameters related to the surface steps. These parameters are the step free energy, the kink creation energy, and the energetic and entropic interactions between steps. This review will demonstrate how a statistical analysis of scanning tunneling microscopy images of stepped surfaces can provide appropriate values of these fundamental energetic parameters. The Si(001) surface is used as a model system. In order to illustrate the significance of these energetic parameters, two morphological surface phase transitions are discussed, namely, the thermal roughening transition and the orientational phase diagram.