The following article contributes to the debate on the concept of abuse of tax law by presenting a functional analysis of general anti-avoidance instruments based on extensive comparative and theoretical prior work. It develops a theoretical model which can be applied to any GAAR or judicial anti-avoidance/anti-abuse doctrine, independently of its particular national or international legal framework. The model is able to explain various methodological and content-related uncertainties concerning general anti-avoidance instruments. In particular, it provides a conceptual framework to categorize common criteria for identifying abusive transactions, e.g., artificiality, taxpayer’s intent, the conflict with legislative intent, business purpose or economic substance considerations, and to understand their function. Furthermore, the functional analysis makes clear why different legal frameworks might suggest different legal answers to the challenge of tax avoidance.