The current article provides a comprehensive survey of quantitative models and calculations of a variety of closely-related contact electrification phenomena that focus on the microscopic origins of the phenomena. Beyond a few ab initio calculations, the most systematic and comprehensive study was carried out by our group using a Hamiltonian formalism. The main focus of this chapter is on three problems we have applied our microscopic theories to: (1) a calculation of electron transfer due solely to quantum-mechanical tunneling, (2) the study of electron transfer coupled with light emission, and (3) a model of tribovoltaics.