In 2021, El Salvador designated Bitcoin as legal tender within that country, payable for all debts, public and private. The move is novel in the realm of cryptocurrency technology, but fits within a time-honored battle for currency control with which every country engages. Bringing Bitcoin into the world of state-supported currencies brings unique challenges from a regulatory and economic standpoint as the imprimatur of a legal tender designation has spillover effects in foreign currency transaction law, foreign exchange finance, payment settlements, and tax law, among other fields. This review article examines the impacts of legal tender designations in these various areas, maps the current regulatory framework of cryptocurrency transactions, and analyzes how future developments may affect these determinations.