Terahertz transitions in solids can be systematically approached by generalizing the semiconductor Bloch equations and the related many-body correlation dynamics. At this level, one realizes the THz field are directly absorbed by two-particle correlations that modify the quantum kinetics of electron and hole distributions. Therefore, a systematic THz analysis must include the quantum kinetics of many-body correlations, that can be treated systematically, e.g., with the cluster-expansion approach. At this level, one can explain and predict a wide range of effects with the same theory, ranging from Drude-like response of plasma to extreme nonlinear effects of excitons.