ESDU 98003 provides a description and overview of the effectiveness (E) - number of heat transfer units (NTU) method for the thermal analysis of two-stream heat exchangers and illustrates its use in design and performance evaluation. It is applicable to all heat exchangers from simple pipework to trains of shell-and-tube exchangers and allows direct performance comparisons of exchanger configurations. The graphical, analytical and empirical data in ESDU 98004 and 98005 are introduced. Step-by-step design and performance calculation methods are laid out (and are illustrated in the design studies in ESDU 98006). The applicability of the E-NTU method in process integration is considered, and the E-NTU and FT-methods are compared. The basis of the method is developed and the assumptions involved are listed. The many features of the method in relation to the practicalities of process plant design and operation are discussed. For example, against a given thermal specification, the method allows the rapid selection of a range of suitable exchanger configurations (and the elimination of others) or the assessment of the performance of an existing exchanger. In design it indicates the minimum terminal temperature difference between the streams and hence the potential for process integration. It calculates the heat loading and so indicates directly whether flow control will be needed prior to an expected in-service level of fouling occurring.