Information on the origin of pollution constitutes an essential step of air quality management as it helps identifying measures to control air pollution. In this document, we review the most widely used source-apportionment methods for air quality management. Using simple theoretical examples we highlight the differences among these methods and explain why and under which circunstances they lead to different results and therefore different conclusions in the context of air quality management. These differences are a consequence of the intrinsic assumptions that underpin the different methodologies and determine/limit their range of applicability. We show that ignoring their underlying assumptions is a risk for efficient/successful air quality management as these methods are sometimes used beyond their scope and range of applicability. The simplest approach based on increments, contributions obtained through receptor models or tagging approaches built in air quality models as well impacts obtained via “brute-force” methods are discussed. The guide is organised as follows: the different source apportionment approaches and their associated properties are presented in Part I, simple examples are introduced in Part II to illustrate the main differences in terms of results while Part III focuses on the fitness-for-purpose aspects of the different methods. Finally Part IV lists and briefly discusses a series of open issues.