Entropy: the degradation of matter and energy in the universe to an ultimate state of inert uniformity. This definition is ultimately the subject of this conference: how to retard, inhibit, or correct the ineluctable degradation of the materials of which our buildings are constructed. All physical materials, including the nominally obdurate stones which we employ in architecture, are subject to this degradation. Under the law of entropy as we now understand it, there is no such thing as a permanent or everlasting material, though each undergoes change at a different rate. The process begins the day the building is completed. It cannot be prevented; it can never be reversed; its rate can only be slowed down. Contemporary scientific knowledge and modern technology permit a range of interventions whereby the entropic process can be controlled.