In this study, the heat transfer behavior in the transition region for plain horizontal tubes under a uniform wall heat flux boundary condition is discussed in detail. In particular, the influence of inlet configuration and free convection superimposed on the forced convection (or mixed convection) at the start and end of the transition region and the magnitude of heat transfer are addressed. The available correlations to predict the heat transfer coefficient in the transition region are reviewed, and their performance are evaluated based on 1290 experimental data points obtained under a wide range of experimental conditions. Appropriate correlations for the mixed and forced convection transition regions are recommended. Finally, a flow regime map for determination of the boundary between forced and mixed convection in horizontal tubes with different inlets is presented.