Various kinds of pores in solid materials are classified into intraparticle pores and interparticle pores according to the origin of the pores. The structural factors of the pores are discussed as well as the methods for evaluation of the pore size distribution with molecular adsorption (molecular resolution porosimetry), small angle X-ray scattering, mercury porosimetry, nuclear magnetic resonance, and thermoporositmetry. Recent progresses in molecular resolution porosimetry for micropores are stressed; this review describes new approaches such as high resolution N2 adsorption, He adsorption at 4.2 K, and molecular simulation for the characterization of porous solids.