This book presents a unified development of the mechanics of shells and plates with emphasis on the fundamental engineering aspects. Initially, the geometrical relationships are presented in a somewhat general form appropriate for both plates and shells. Following the geometric description of the surface and the consideration of the equilibrium, we introduce a first logical simplification, the membrane theory of shells. After a further theoretical exposition of linear deformations, constitutive relationships, and energy principles, we present the flexural theory of plates and the bending theory of shells, including elastic stability. While this sequence postpones the introduction of the complete theory of plates until much of the theory of shells is covered, it is logical, consistent and efficient.