An overview is given of the changes that occur in the pore size distribution of dried clay as it is transformed into a ceramic product by firing at high temperatures. In addition to the changes that take place in the total porosity or density of the product, the pore size distribution shifts from a fine pore structure to a structure with larger-diameter pores. This process takes place in several - quite different - ways, depending on the nature of the clay used. Other topics discussed include the influence of pore size distribution on the various properties of different ceramic products; the effect on frost resistance; the relationship with the impermeability of ceramic roof tiles, and the relationships, still to be verified, between salt efflorescence on brickwork and the pore size distribution of brick and mortar. 4 refs.