The importance of seismic rehabilitation became evident with 1992 Erzincan Earthquake, after which a large number of reinforced concrete buildings damaged in recent earthquakes required strengthening as well as repair. In the studies related to rehabilitation, it has been realized that inadequate lateral stiffness is one of the major causes of damage in reinforced concrete buildings. Recently, economical, structurally effective and practically applicable seismic retrofitting techniques are being developed in METU Structural Mechanics Laboratory to overcome these kinds of problems. The strengthening technique proposed in this thesis is on the basis of the principle of strengthening the existing hollow brick infill walls by using high strength precast concrete panels such that they act as cast-in-place concrete infills improving the lateral stiffness. Also, the technique would not require evacuation of the building and would be applicable without causing too much disturbance to the occupant. For this purpose, after two preliminary tests to verify the proper functioning of the newly developed test set-up, a total of fourteen one-bay two story reinforced concrete frames with hollow brick infill wall, two being unstrengthened reference frames, were tested under reversed cyclic lateral loading simulating earthquake loading. The specimens were strengthened by using six different types of precast concrete panels. Strength, stiffness, energy dissipation and story drift characteristics of the specimens were examined by evaluating the test results. Test results indicated that the proposed seismic strengthening technique can be very effective in improving the seismic performance of the reinforced concrete framed building structures commonly used in Turkey. In the analytical part of the study, hollow brick infill walls strengthened by using high strength precast concrete panels were modelled once by means of equivalent diagonal struts and once as monolithic walls having an equivalent thickness. The experimental results were compared with the analytical results of the two approaches mentioned. On the basis of the analytical work, practical recommendations were made for the design of such strengthening intervention to be executed in actual practice.