This paper provides the state of the art in methods to detect damage in structural and mechanical systems by PZT sensors, which include impedance methods and lamb wave propagations. The basic idea behind these technologies is to use impedance of PZT embedded on surface or lamb wave propagations. A brief overview of research work on experimental and theoretical studies on various structures and mechanical systems is descried. Past, current and applications of this technology in actual engineering systems are summarized. Finally, desirable developments for further advancement of this field are discussed.