The importance of laminar flow forced convection theoretical solutions for the design of compact heat exchangers has been well realized. Many of these theoretical solutions published in the literature are difficult to interpret by a designer, because thermal boundary conditions used in the analysis are generally not defined clearly and consistently. The objective of this paper is to clarify this situation by a systemized presentation. Starting with a general classification, nine specific thermal boundary conditions are categorized for singly connected ducts. At the end, the most useful fully developed laminar flow solutions for some important duct geometries are summarized for the designer.