Heat exchangers have been classified in various ways. Double pipe, and shell and tube heat exchangers are most widely used. This chapter deals with heat exchanger fundamentals and design of heat exchangers. Heat transfer equation for double pipe heat exchanger has been presented and log mean temperature difference (LMTD) has been defined in Part 1 of the chapter followed by derivations of equations of LMTD for parallel flow and counterflow exchangers. In Sect. 14.1.4, graphs of correction factor for calculation of LMTD for other flow arrangements have been presented. Basic treatment of effectiveness-NTU method for counterflow and parallel flow heat exchangers is given in Sect. 14.1.5. Graphs for determination of effectiveness for other flow arrangements have been given as function of NTU in Sect. 14.1.6. In second part of the chapter, basic considerations for heat exchanger design have been presented followed by defining of fouling factor, and clean and design overall heat transfer coefficients. An illustrative example of double pipe heat exchanger design has been given.