The Base Isolation technique and its benefits in reducing the transmitted earthquake energy into a structure has gained increasing recognition during the last two decades. This recognition is indicated by the application of Base Isolation systems to a large number of bridges, several multistorey buildings and some power plants in countries which have high seismic risk. Unfortunately, the currently available design procedures, especially for multistorey structures, seem inadequate and too restrictive and as a result present practice still relies upon a series of deterministic time history analyses which are not only impractical for design purposes but appear unable to give the designer a clear insight into the seismic behaviour of the multistory structure. This research is carried out to investigate in more detail the effects of various structural parameters and ground motion characteristics on the seismic response of Base Isolated multistorey structures. It also reviews the shortcomings of the current design methods. The results are then used to develop two simplified analysis methods for practical design. The first method which is called the Code-Type approach can be used to accurately estimate the inertia forces, not only at the level of the isolation devices but throughout the height of the multistorey structure. It is recommended for use as a preliminary design tool or even a final design tool for simple Base Isolated multistorey structures. The second procedure which is based on the Component Mode Synthesis method is suggested for final design purposes of more complex Base Isolated multistorey structures. This method enables the designer to evaluate the effects of the isolation devices on the contribution of each mode of vibration to the total response of the structure.