U-type return bends are widely employed in the condenser and evaporator of the refrigerating systems. The presence of curved tubes will induce secondary flow due to the contribution of the centrifugal force. As a consequence, heat transfer and fluid flow characteristics change considerably. In the two-phase flow application, the flow pattern in the return bend is dramatically affected by the vortices of the secondary flow. The induced flow disturbance would greatly increase the pressure drop and heat transfer in bend and its downstream. Since flow patterns are intimately interrelated to two-phase heat transfer and pressure drop, it is essential to identify the actual fluid flow characteristics subject to the influence of curved tubes. The objective of this study is therefore to compile and to review the recent progress of this subject. It is found that the presence of return bend gives rise to many unique fluid flow characteristics especially for those in two-phase flow such as flow reversal, freezing slug, and change of flow patterns. These interesting phenomena will certainly affect the thermofluids characteristics of the heat exchanging equipments. Relevant influences on the frictional characteristics for single-phase and two-phase flows are presented. Correlations applicable to single-phase and two-phase flows are compared with the measurements. In general, the predictive ability of these existing correlations are in the range of 10~20%.