A theoretical discussion of the principles involved in the fracture of concrete under stress is presented. The strength determining property for all types of loads is the critical strain energy release rate. The elastic energy is transformed mainly to surface energy, but the new surfaces include a multitude of microcracks much larger in total area than the main crack. The increase of the microcracked zone and the heterogeneity of concrete contribute to its strength. The basic difference between tensile and compressive fractures is that, in the former, the rate of strain energy release (i. e., the driving force) increases with crack length, whereas, in the latter, it is constant. This makes compression a more controlled type of fracture than is tension. In tension, the first crack is also the fatal one: hence, the stress-strain curve is almost linear. In compression, fracture is preceded by a process of progressive cracking that accounts for the higher strength and the greater curvature of the stress-strain relationship.