It has been established that masonry infill walls affect the strength, stiffness and ductility of infilled frame structures. However, when designing such structures, it has become common practice not to include the existing infill walls in the structural analysis model as these elements are considered to be essentially non-load bearing. In doing so, the stiffness and strength contribution of the latter elements as well as their interaction with the members of the surrounding frame are ignored. Based on the available, published, experimental and numerical data, it becomes clear that the infill walls have a significant effect on the overall structural performance of the frames. Moreover, based on the above data it has been established that the infill walls usually act as diagonal compression 'struts' in the plane of the frame, resulting in an increase of the overall stiffness of the structure and in a reduction of its natural period, which in turn affects the distribution and intensity of the inertia loads generated during seismic excitation, as well as the distribution of the internal actions developing within the structural elements. Although the introduction of infill walls usually results in an overall increase of the seismic capacity and stiffness of the in-filled frame, it may also cause the development of stress concentrations in certain regions of the structure (e.g. the end regions of elements) leading to localized cracking or even unexpected forms of failure, which may potentially have a detrimental effect on the overall response of the frame.Over the last three decades a large number of experimental investigations have been carried out in an attempt to quantify the effect of infill walls and panels on the structural response of frame structures. During these investigations, scaled models of one, two and even three storey bare (with no infill walls) or in-filled frames were tested. These specimens were subjected to monotonic or cyclic static loading or seismic excitation through shake table testing. The experimental information obtained (concerning deformation profiles, crack patterns, displacements, acceleration and base shear time histories, modes of failure etc) reveal that the introduction of infill walls into frames results -in general-in an overall increase in seismic capacity and stiffness. However, one needs to exercise caution when dealing with frames with brittle material (i.e. concrete), with irregular shapes or with openings in the infill walls, since the development of stress