This paper reviews our systematic and exhaustive studies on the lead zirconate titanate doped with dopants using ab initio density functional theory calculations in order to understand the mechanisms behind the dopings. Different candidates of dopants were selected by screening the periodical table of elements. In our studies, group VA, VIA elements (B-site donors), group IIA elements (A-site donors), group IIIB elements (B-site acceptors), and group VB elements (A-site donor, B-site acceptor/donors) are investigated as dopants in PZT. We found that there exist different mechanisms behind the improved ferroelectric properties, especially the fatigue behaviors. For donors doping, diluted oxygen vacancy concentration and reduced electronic suppression of polarization contribute to the fatigue-free behaviors of donor doped PZT. On the other hand, for acceptor doping, acceptor-oxygen-vacancy-acceptor clusters are energetically preferred, which greatly reduce the oxygen vacancy mobility and the domain pinning effects. We expect that this study could provide important information for the experiments on PZT-based materials.