The book provides design engineers an elemental understanding of the variables that influence pressure drop and heat transfer in plain and micro-fin tubes to thermal systems using liquid single-phase flow in different industrial applications. It also provides design engineers using gas-liquid, two-phase flow in different industrial applications the necessary fundamentals of the two-phase flow variables. The author and his colleagues were the first to determine experimentally the very important relationship between inlet geometry and transition. On the basis of their results, they developed practical and easy to use correlations for the isothermal and non-isothermal friction factor (pressure drop) and heat transfer coefficient (Nusselt number) in the transition region as well as the laminar and turbulent flow regions for different inlet configurations and fin geometry. This work presented herein provides the thermal systems design engineer the necessary design tools. The author further presents a succinct review of the flow patterns, void fraction, pressure drop and non-boiling heat transfer phenomenon and recommends some of the well scrutinized modeling techniques.