This chapter reviews the traditional optimal sensor placement (OSP) methods. In the context of structural damage detection, the basic principle of the OSP is that the sensors should be placed appropriately so as to effectively capture the variations in structural characteristics caused by structural damages. By using different criteria or objective functions, a number of approaches have been developed for the OSP and most methods utilise the information obtained from the frequency domain. Based on the Kalman filter algorithm, the chapter addresses the problem of placing multi-type sensors, which include accelerometers, displacement transducers and strain gauges, in a structure with the objective of the best reconstruction of structural responses. To assess the feasibility and accuracy of the proposed dual-type sensor placement method, dynamic tests on a simply supported overhanging beam were conducted. The numbers and locations of both strain gauges and displacement transducers can then be determined by applying the proposed dual-type sensor placement method.