Customary international law is law that “results from a general and consistent practice of states followed by them from a sense of legal obligation.” “International agreements create law for states parties thereto and may lead to the creation of customary international law when such agreements are intended for adherence by states generally and are in fact widely accepted.” Does customary international law (CIL) exist in tax? There are over 3,000 bilateral tax treaties, and they are about 80% identical to each other, but do they create CIL that binds in the absence of a binding treaty, like for example the Vienna Convention on the Law of Treaties binds the US, which has not ratified it? This chapter will argue that the answer is yes, using four examples: jurisdiction to tax, the permanent establishment (PE) threshold, the arm’s length standard, and non-discrimination.