Questions of size effect and scaling on the integrity of structures have been around since at least the time of Leonardo da Vinci. Bazant (civil engineering and materials science, Northwestern U.) sketches the history of size effect studies before exploring size effect on fracture and crack mechanics in a number of materials. He explores applications of the known size effect law for the measurement of material fracture properties and the modeling of the size effect by the cohesive crack model, nonlocal finite element models, and discrete element models. Applications to quasibrittle materials, including concrete, fiber composites, sea ice, rocks, and ceramics are presented. The role of size effect in some famous structural catastrophes is then examined.