A brief outline of the application of fractals to porous solids is given. The variation of porosity and surface area as functions of particle size and pore size are discussed. For F-20 Alcoa alumina most of the surface is contained in pores of radius near 20 A. However, fractal behavior is observed if probe molecules are used that are too large to fit into pores of radius near 20 A. For some zeolites, fractal behavior can be observed if probe molecules are used that are smaller than the largest openings in the microcrystalline structure. Neither conventional materials like alumina, nor zeolites, should be considered to be fractals (approximants), but each may sometimes exhibit fractal behavior over a certain range of probe sizes.