The interaction of light with selected solids is firstly presented in relation with photochemical processes in heterogeneous systems. The principle of the heterogeneous photocatalytic method and a survey of the relevant literature are also presented. These include a survey of typical photocatalytic reactions observed on heterogeneous dispersions of photocatalysts based on powdered semiconductor materials, in an attempt to represent the wide applicability and versatility of this method. Particular emphasis will be placed on the field of preparing heterogeneous photocatalysts. The influence of various parameters (as particle size, texture, bulk and surface perturbation, etc.) which govern the activity of a given photocatalyst material will be indicated and discussed. Results obtained in our laboratory will be recalled to illustrate some aspects within the scope of this article.