This chapter considers potential legal challenges to digital taxes in the European Union (EU) and United States (US). Because they apply only to companies with very large turnovers and to very narrow revenue streams, digital taxes may, as applied, discriminate on the basis of nationality, which could violate the fundamental freedoms under EU law. For the same reason, digital taxes may discriminate against international or interstate commerce, which could violate the dormant Commerce Clause doctrine of the US Constitution. Additionally, federal law in the US that forbids taxing electronic commerce more than other types of commerce may preclude state digital taxes, as one state court in Maryland has already held. To reduce the likelihood of digital taxes being held unconstitutional or in violation of the Treaty on the Functioning of the European Union, US and EU Member States should consider: (1) lowering the revenue thresholds for triggering digital taxes and (2) applying them to a broader base. It seems that ever since the EU Commission introduced its proposal for a digital tax, nearly every country has considered, proposed, or even adopted a digital tax. Digital taxes are typically gross taxes that apply to revenue from specified sources including, for example, digital advertising, the provision of a two-sided marketplace, online subscriptions, the sale of user data gathered on digital interfaces, and similar activities. Focusing on the US and the EU, this chapter describes potential legal challenges to digital taxes and how EU Member States and US states might craft their digital taxes to avoid legal infirmities.