In Chap. 1, we have introduced the concept of convection through a phenomenological description by introducing h, the convection heat transfer coefficient. The heat transfer coefficient was introduced through the so-called “Newton’s law of cooling”. In many problems encountered in conduction heat transfer, we have made use of a suitable “h” value to describe what happens at a boundary between a solid and the ambient fluid. However, no effort was made to describe the basis for choosing a particular value of h. In what follows we would like to calculate h by using fundamental heat transfer principles that are involved in the case of a flowing fluid.