The principles of condensation in horizontal tubes are presented with special attention to the prediction of local heat transfer coefficients as a function of the local flow pattern. The influence of flow pattern, especially flow stratification or not, is shown to be very important in accurately predicting local condensation heat transfer coefficients. In addition to condensation of pure vapors (and azeotropic mixtures), condensation of zeotropic mixtures are also addressed along with condensation of superheated vapor and the subcooling of condensate. Principle focus is on the flow pattern model of El Hajal et al. (2003) and on the flow pattern based heat transfer model of Thome et al. (2003). For additional models on intube condensation, the reader is referred to Volume 2 of Set 1 of this series by Garimella and Fronk (2015).