A series of chiral tetradentate diaminodiphosphine ligands (PNNP-type) have been synthesized and characterized. These ligands contain two "soft" phosphorus atoms and two "hard" nitrogen atoms, which display the rich coordination chemistry and easily modify the steric and electronic properties of the resulting metal complexes. Based on these polydentate mixed-P,N ligands, the chiral PNNP-Ru, PNNP-Rh and PNNP-Fe complexes were prepared and used as catalysts in the asymmetric transfer hydrogenation of aromatic ketones with up to 99% ee and even the molar ratio of ketone to catalyst up to 10000:1. The catalytic reaction mechanism of ATH will be discussed. The PNNP ligands extensively used in asymmetric epoxidation, cyclopropanation and kinetic resolution by other researchers and us are also reviewed.