The substitution on either the A-or B-site of ferroelectric perovskites by aliovalent elements has a profound influence on their properties. Donor doping "softens" ferroelectrics, whereas acceptor doping "hardens" them. The charge compensation mechanisms are reviewed, as well as the models describing their effects. The focus of this review is doped-BaTiO 3 , a model lead-free ferroelectric. The effects of aliovalent doping on its dielectric, ferroelectric, and piezoelectric properties are reviewed and illustrated in the case of Cu (acceptor) doping.