Pores are in aerogels essential. Experimentersoften usethe nitrogen adsorption measurement technique and derive from the desorption curve the pores' size distribution assuming cylindrical pores and the Kelvin equation to be applicable.A description of the pores is difficult and the situation is not comparable with, for instance, closed cell foams. Scanning electron microscopy gives an imagination of the particles or fibrils and thus also the pores. Nevertheless, there are simple measures for pore sizes possible, which are well defined in stereology, namely the mean free distance between particles or fibrils in a network and the next nearest neighbour distance. In addition, scattering methods allow us to extract chord lengths in pores and the solid phase assuming a suitable model of the two-phase structure. The experimental techniques such as the BJH model and thermoporosimetry are discussed and the basic equations derived. The theoretical models are compared with experimental results for different aerogels.