In this first project of a research program, initiated to investigate the behaviour of coupled shear walls subjected to seismic type lateral loading, the strength and behaviour of the coupling beams is examined. The limitations of the elastic "laminar analysis" are examined and the technique is extended so as to predict the elastoplastic behaviour of a cracked shear wall structure and, in particular, the ductility requirements for coupling beams. Twelve approximately ¾ full size relatively deep reinforced concrete coupling beams, with various aspect ratios and web steel contents, have been tested under static one-way and near-ultimate cyclic loading. The experiments, which attempted to trace every aspect of the behaviour, revealed numerous features not encountered with normal shallow reinforced concrete beams. Shear and flexural failure mechanisms were identified and the nature of deterioration with cyclic loading was observed A theoretical approach is suggested which satisfactorily approximates the loss of stiffness of coupling beams after diagonal cracking. Experiments and theory both indicated a loss in excess of 80%. The significant findings of the experimental investigation have been translated into design recommendations for coupled shear wall structures.