Two-phase flow of gases and liquids or vapors and liquids in pipes, channels, equipment, etc. is frequently encountered in industry and has been studied intensively for many years. The reliable prediction of pressure drop in two-phase flow is thereby an important aim. Because of the complexity of these types of flow, empirical or semiempirical relationships are only of limited reliability and pressure drops predicted using leading methods may differ by up to 100%. In order to improve prediction methods, this work presents an experimental and analytical investigation of two-phase pressure drops during evaporation in horizontal tubes. The goal of the experimental part was to obtain accurate two-phase pressure drop values over a wide range of experimental conditions. The existing LTCM intube refrigerant test loop has been modified and adapted to the new test conditions and measurement methods. Two new test sections have been also implemented into the modified test rig. The new test section consists of two zones: diabatic and adiabatic. This configuration allows tests to be run that obtain experimental two-phase pressure drop values under diabatic and adiabatic conditions simultaneously. The experimental campaign acquired 2543 experimental two-phase pressure drop values. Based on a comprehensive state-of-the-art review and comparison with two-phase frictional pressure drop prediction methods, it is proven that none of these methods were able to accurately, reliably predict the present experimental values. In the second part of this work, an analytical study was undertaken in order to develop a new two-phase frictional prediction method. It has been shown in the literature that the so called "phenomenological approach" tends to provide more accurate and realistic predictions as the interfacial structure between the phases is taken into account. Based on that, a phenomenological flow pattern approach was chosen in the present study. The recent Wojtan-Ursenbacher-Thome [155] map was chosen to provide the corresponding interfacial structure. The new model treats each flow regime (i.e. interfacial structure) separately and then ensures a smooth transition in between, being in agreement with the experimental observations. Another important feature of the proposed model is that it matches the correct limits at x = 0 (single-phase liquid flow) and x = 1 (single-phase gas flow). Based on a statistically comparison, it is concluded that the new two-phase frictional pressure drop model based on flow pattern map successfully predicts the new experimental data. The present work completes the fourth basic step in LTCM's flow pattern based work on two-phase flow and heat transfer inside horizontal round tubes: (i) generalized flow pattern map, (ii) flow boiling heat transfer model, (iii) convective condensation model and (iv) two-phase frictional pressure drop model.