Ferroelectric materials are one of the smartest materials known to us, which have multiple functional properties, including piezoelectric, dielectric and pyroelectric characteristics. Since functional properties are usually associated with response agilities of materials to external stimuli, better functional properties may be created if one could make the crystal structure or mechanical structure of materials less inert As discussed in this chapter, various defects, including vacancies, aliovalent dopants, domain walls, grain boundaries, interstitial defects, surfaces, etc. have been introduced into ferroelectric materials to weaken the stability of crystal structure or domain structure so that some intended functional properties can be greatly enhanced. In fact, any ferroelectric material used as a functional material contains some types of defects. These defects may be chemically introduced through doping or are being physically created through thermal processes or domain engineering. Understanding the role of each type of defect can help us use defects properly to our advantage in designing better functional materials and in creating smaller and more advanced electric or electromechanical devices that can further facilitate our life.