In the high temperature superconductors the critical temperature is intimately connected to the electronic charge-carrier concentration. The absolute values of this quantity and its dependencies on the controlling parameters temperature, oxygen partial pressure and doping content, is phenomenologically determined by the thermodynamics and kinetics of reactions involving point defects. Due to the low mobility of the ionic constituents the behavior at elevated temperatures is particularly important. Here, the kinetics are controlled by the mass transport parameters, essentially by the ionic conductivity. The scientific discipline whose objective is precisely described in the above terms is called defect chemistry. It quantitatively deals with the nature of excess and lacking native ionic or electronic particles as well as with foreign species. It describes the thermodynamics and kinetics of their formation, annihilation, interaction and motion in the phase under consideration and thus determines the chemistry within this phase.