In this course, students study a challenging and exciting field of a kaleidoscopic nature: a classic right of overlap that repeals national systems, a right of contract that expresses an agreement between sovereign states and a right of direct effect that can be invoked by individuals against the latter, all at once. In this context, State claims to tax run up against the gradual multi-location realities in personal and corporate lives, which generate multiple possibilities for taxation and create opportunities for sophisticated planning on a cross-border scale. This feeds on arbitrage and competition among the various systems. The fundamental matrix is the OECD Model Convention, which will be systematically studied and enriched by national case law.