This chapter presents the theory and the main types of liquid-liquid, liquid-gas, and gas-gas heat exchangers. The modeling of the most basic double-tube heat exchanger is presented, and its working equations are based on the logarithmic mean temperature difference, \(LMTD\) , for parallel flow and counterflow heat exchanger configurations. In parallel flow, the two fluids circulate in the same direction, while in the counterflow configuration, they circulate in opposite directions. To solve the cases of more complex heat exchangers than the double tube, the F solution method is presented as a parameter that corrects the \(LMTD\)  for more complex configurations. Graphs and equations for various configurations are presented. In the case of the inlet and outlet temperatures not being specific or known, the most common method used is the effectiveness-number of transfer units, ε—NTU. The theoretical basis of the method is developed in terms of the ratio between the thermal capacities of the two fluids. Graphs for various heat exchanger configurations are presented. The chapter concludes with an appropriate discussion on irreversibily in heat exchangers.