This chapter introduces and describes the terminology of material flow analy sis (MFA). The exact definition of terms and procedures is a prerequisite for generating reproducible and transparent results and for facilitating communication among users of MFA. The definitions as developed by Baccini and Brunner (1991) during the 1980s are employed. As described in Chapter 1, Section 1.1.3, there have been several parallel developments of MFA, resulting in diverse usage and meanings for single terms. For example, the terms substance and materials have been applied to substances as well as goods, and sometimes several names are used for the same object. Needless to say, the presence of such a babel within the global MFA community complicates the application and usage of MFA, especially for the new user. Differences appear not only in the employment of single terms but also on a higher conceptual and methodological level. In this chapter, these aspects are addressed and discussed whenever appropriate. The authors’ intention is to contribute to a generalized terminology in MFA.