During the last two decades, the literature devoted great attention to industrial symbiosis (IS) as an effective strategy to achieve environmental, economic, and social benefits. Accordingly, a wide range of numerical indicators – highly different among them for scope, definition, purpose, and applications – have been developed, to characterize and measure IS. The paper proposes a taxonomy of these indicators with the aim of facilitating their adoption and proper usage in practice. The taxonomy is developed on the basis of a literature review and is addressed to answer three main questions: (1) what to measure, (2) where to measure, and (3) how to measure. This offers a clear picture of available relevant IS indicators in terms of purpose, context, and methodology.