Three fundamental experiments on multi-particle interactions in hard-sphere systems are presented in this thesis. In the first experiment, a mixture of two monodisperse colloids with different particle diameters, but the same particle composition, is studied using diffusing-wave spectroscopy. We find that both the particle structure and the hydrodynamic interactions between large and small balls are well modeled by existing theories. These theories, however, do not comment on the observed diffusion of particles in complex solutions over distances larger than a particle diameter. Incidental observations during this experiment led to the second experiment, the study of the phase diagram of binary hard-sphere mixtures of colloidal particles. In the process of mapping this phase diagram, driven entirely by entropy, a novel surface phase was discovered. This surface phase is explained geometrically in terms of the entropy of the small particles in the suspension. In the final experiment, the diffuse model of photon transport is extended to create a new technique called Diffuse Transmission Spectroscopy (DTS) for exploiting the wave vector dependent structure of colloidal suspensions. DTS uses the wavelength dependent transmission of a slab of colloid to probe interparticle structure. The diffuse transmission spectrum is shown to be useful for measuring particle diameter and differences in interparticle potential in dense, undiluted systems, as well as for testing the validity of structural models.