The major part of this thesis is concerned with the determination of the charge densities of small organic molecules, using both experimental and theoretical methods. A smaller section details the theoretical determination of the spin densities of some sulfur-containing model systems. Chapter 1 traces the historical development of the theory of multipole modelling from the development of the first aspherical scattering factors. The practical implementation of the method. the experimental requirements and subsequent treatment of the data are also discussed. Chapter 2 gives a summary of ab burin molecular orbital methods. Particular attention is given to the way in which the atomic and molecular properties which are of interest to the chemist are obtained from the calculated wavefunction. Throughout the relevant results chapters the experimental and theoretical charge densities are analysed using Bader's theory of Atoms in Molecules. Chapter 3 gives an outline of this analytical approach. Chapter 4 reports the results of experimental and theoretical charge density studies of 3,3.6,6-tetramethyl-s-tetrathiane. Particular attention is given to the strength of the sulfur-sulfur bond. An ab wino study is made of the relative stabilities of the three possible conformers of the molecule. An experimental charge density study of hexakis(mercaptomethyl)benzene is reported in Chapter 5; evidence is found for the existence of intermolecular SH...S hydrogen bonding. Chapter 6 contains experimental and theoretical results for the charge density study of a salt of the antifungal compound E-tetraethyl-1.4-diammoniumbut-2-ene. A quite different area of physical chemistry is investigated in Chapter 7. The results of a series of calculations of the hyperfine coupling constants of muonium in sulfur environments are presented. This work was carried out in order to try to identify the muonic radical species which is observed by muSR when muons are introduced into elemental sulfur.