The fracture of concrete is reviewed. Tests on notched and unnotched beams made from concretes containing aggregates of different sizes and types are described. A simple model is proposed to elucidate and formalize the role of the aggregate particles in a brittle matrix. It is concluded that aggregate particles cause cracks to form in a concrete matrix at a lower stress than that at which the matrix would crack if it contained no aggregate. It is also concluded that aggregate particles impede the extension of matrix cracks.