Masonry buildings have been, and remain, a popular form for economically enclosing space. Whereas such buildings are safe under gravity loads, most are vulnerable to horizontal loads due to earthquakes. Observations following an earthquake and experimental programs have shown that piers between openings are the most vulnerable part of a masonry building, and that the failure of such piers is due in the majority of cases to shear (or diagonal tension). Accordingly, the study described concerns the seismic response of reinforced masonry piers that exhibit a shear mode of failure. The study consists of two parts. First the results of an experimental program on reinforced masonry piers under cyclic lateral loads simulating seismic excitation are presented. Then several code provisions governing the seismic design of masonry are evaluated in view of the experimental observations, and a new seismic shear design concept is proposed for reinforced masonry piers and walls where the design strength is defined as the onset of diagonal web cracking. Shear reinforcement is considered as a mechanical component that provides postcracking ductility. Validity of the proposed concept is verified by comparing the computed design parameters with the available experimental data.