The most efficient earth abundant, non-toxic thin film multelemental PV devices are fabricated from Cu, Zn, Sn, S and Se, with the chemical formula of Cu2ZnSn(S x Se1-x)4 (CZTS,Se). This material has enjoyed relatively rapid increases in efficiency from its inception to its present-day power conversion efficiency of 12.6%. But further increases in efficiency have been hampered by the inability to substantially increase Voc, the open circuit voltage. In this review article we will discuss the fundamentals of this important kesterite material including methods of film growth, post growth processing and device fabrication. Detailed studies of the properties of CZTS,Se including chemical, structural and electronic as well as full device electrical characterization have been performed in an effort to coax out the critical issues that limit performance. These experimental studies, enhanced by density functional theory calculations have pointed to fundamental bulk point defects, such as Cu-Zn antisites, and clusters of defects, as the primary culprits in limiting Voc increases. Improvements in device performance through grain boundary passivation and interface modifications are described. Exfoliation of functioning solar cells to expose the back surface along with engineering of new back contacts designed to impose electrostatic fields that drive electron-hole separation and increase Voc are discussed. A parallel route to increasing device performance by alloying Ag with CZTS,Se in order to inhibit Cu-Zn antisite defect formation has shown significant improvement in material properties. Finally, applications of high S (and hence higher Voc) CZTS,Se based devices to energy harvesting for 'Internet-of-Things' devices is discussed.