First place, in this work the way of scientific investigation is cleared, based always in a plural scientific approach with a strong link to systematic and integrative methods towards various species of science. The first chapter deals with the clarification of the general premises of the work, based on law as a linguistic phenomenon, studied by Language Theories, Law Logics (aggregated on logic-semantic constructivism) and Niklas Luhmann''s System''s Theory. In the second chapter, avoidance and anti-avoidance rules are shown as general structures of law, in the lights of legal unity and System''s Theory. The third chapter deals with the fractal and asymmetric classification of the anti-avoidance rules, adding an example of these rules outside the tax realm. The fourth chapter deals with the specificities of this new approach of the anti-avoidance tax rules and its developments and effects, analyzing also the differences to tax planning and tax evasion. The fifth and last chapter deals with anti-avoidance (especially its criteria) in different legal systems, such as in Brazil, Italy, USA and in international tax law, including the treaties to avoid double taxation and EC law.