Liquid toners have been used in the development of electrostatic images in digital printing since the midfifties. In this article, several models are discussed that were introduced to explain the processes responsible for charging toner particles. The acid-base models had to be extended by equilibrium models describing dynamic dissociation, adsorption and desorption. The important role of water traces in non-aqueous media for the dissociation of molecules and stabilization of ions was investigated. Later, electrical conditions were considered to explain specific high field effects by additional charge generation and relaxation processes. The focus of this article is on the models developed for the charging mechanisms. The enormous progress in measurement techniques that was necessary for improving and, in some cases, correcting the models is not described in detail here nor are models concerning practical issues in printing.