The behaviour of reinforced concrete flat plate structures, subjected to various combinations of vertical and horizontal loads, was studied. Being mainly experimental, the investigation consisted of (i) The development and the design of the instrumentation systems to obtain the appropriate information; (ii) the observation of the behaviour at various stages and under various load combinations, and (iii) the evaluation of the experimental data which resulted in a suggestion for a method of analysis applicable to the design of structures of this particular type. Two large scale test structures, consisting of a 21 ft. x 21 ft. and 4 in. thick plate supported on nine columns spaced at 10 ft. centres, were designed, instrumented and tested. The studies of the behaviour of these two test structures concentrated on two main aspects of the problem: i. The general flexural behaviour of the test structure, under various load combinations, ii, The local behaviour of various types of slab-column connections, with particular reference to the punching shear strength problem. For both of the above aspects, various methods of analysis are applied; and compared with the experimental findings. A number of conclusions are drawn from these comparisons as well as from the observations. Some suggestions are also made for further research on the subject.