This chapter covers flow pattern based models for evaporating flows inside plain horizontal smooth tubes. First, the various fundamental elements of two-phase flows are presented, which are the building blocks of flow pattern based models. Then, the flow pattern map of Wojtan et al. (2005b) is presented and described. Next, the flow pattern map based boiling heat transfer model of Wojtan et al. (2005c) is presented and then finally the flow pattern based frictional pressure drop model of Da Silva Lima and Thome (2012c) is described. For all presented models, a parametric study is then provided, including graphical outputs that will help the reader understanding and implementing these models.