An experimental Low Energy Electron Diffraction study of the Si(100) surface. The principles of dynamical LEED theory, Constant Momentum Transfer Averaging and Auger electron emission and its angular dependence are discussed. The experimental systems used for LEED and AES measurements are described together with the specimen cleaning procedures used. It is found that argon ion bombardment followed by annealing is the most effective method of obtaining a clean well ordered silicon surface. The familiar Si(100)(2 x 1) clean surface structure is observed. A new surface phase, the S1(100)(1 x 1)H structure, produced by adsorption of atomic hydrogen, is described. Data is presented for the angular dependence of Auger electron emission from these surfaces. This method is shown to be of little value in structure determination in this case. A comprehensive set of LEED Intensity-Energy spectra for the (2 x 1) and (1 x 1)H structures is presented. It is shown that LEED data for the (1 x 1) surface provide a valuable test case for theoretical models of low energy electron scattering from silicon. The (2 x 1) data furnish an experimental foundation for structure determination of this reconstructed phase. CMTA analysis of the LEED data indicates that the (1 x 1)H structure is probably bulk-like, with a top silicon layer spacing contracted by ≈ 32. For the (2 x 1) surface the analysis indicates that either the reconstruction is deep or that CMTA is ineffective in eliminating multiple scattering peaks in this case. The latter explanation clearly augurs ill for the future value of CMTA in surface structure determination.