A new category of piezoelectric ceramics with very high electromechanical coupling was discovered in a lead zinc niobate-lead titanate solid solution in a single crystal form. The maximum coupling factor k(33) reaches 95%, which corresponds to the energy conversion rate twice as high as the conventional lead zirconate titanate ceramics. This paper reviews the previous studies on superior piezoelectricity in relaxer ferroelectric : lead titanate solid solutions and on the possible mechanisms of this high electromechanical coupling.