The thesis deals with a number topics concerning the equilibrium properties and the phase behavior of colloidal suspensions, a class of physical systems that have attracted a broad interest during the last few decades, serving both as a valuable test bench for ideas in condensed matter physics and as building blocks for novel materials. An emphasis is placed on the notion of effective interaction, which enables the construction of simple models that are studied by use of theory and computer simulation. The questions addressed include the thermodynamics of particles interacting through a short-ranged attraction, fluctuation-induced (critical Casimir) forces, and the effect of polydispersity on the freezing transition of charged colloids
