A critical review of evaporation in plain, horizontal tubes is presented, focusing primarly on flow boiling correlations. Emphasis is placed on analyzing the integrity of currently accepted ideas and methods, identifying the weaknesses in current design methods, noting the advances in two-phase flow research that can be applied towards improvement of flow boiling correlations, and providing some guidelines for future correlational improvements. In particular, numerous aspects related to the boiling suppression factor S, the liquid heat transfer coefficient hL, the two-phase convection multiplier F, and the flow stratification parameter FrL are not consistent with the mechanics of flow boiling in horizontal tubes.