Cu 2ZnSnS 4 (CZTS) is one of the most promising quaternary absorber materials for thin-film solar cells. Light to electricity conversion efficiencies in CZTS devices have recently reached 9.6%, making it a competitive and more sustainable replacement for existing CdTe and Cu(In,Ga)Se 2 (CIGS) thin-film technologies. We review our recent insights into the structural, electronic and defect properties of this topical material. We have found that the stable crystal structure of CZTS is kesterite, which is derived from the ternary chalcopyrite structure. Examination of the thermodynamic stability of CZTS reveals that the stable chemical potential region for the formation of the stoichiometric compound is small. Under these conditions, the dominant defect will be p-type Cu Zn antisite, which has an acceptor level deeper than the isolated Cu vacancy. The dominant self-compensated defect pair is [Cu Zn+Zn Cu], which leads to the formation of various polytype structures. We propose that to maximize the solar cell performance, growth of CZTS under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided. This theoretical guidance can provide new directions for improving the conversion efficiencies of kesterite based solar cells.