While carbon tax measures have not yet met with success at the federal level in the United States, proposals for carbon taxes emerged in a handful of states in 2015 and 2016. The proposals address the shared challenge of climate change, but each has its own unique features and setting. Drawing on proposals in Oregon, Massachusetts, Vermont and Washington as case studies, this chapter explores how state constitutions can affect the design of state-level carbon taxes and their legislative route toward enactment. For example, the Oregon constitution imposes limits on tax rates and use of the revenue when taxing certain fossil fuels. The constitutions in three of the four states require that some types of revenue measures must originate in the legislative House of Representatives, not the Senate, raising the question whether carbon taxes can be designed in a manner that will avoid this procedural constraint. In Washington, the carbon tax proposal came forward as a ballot initiative that went to voters in the general election, following a procedure permitted under the state constitution. These case studies serve as an important reminder of how constitutional provisions that were not created with climate change in mind can influence the design features of subnational carbon taxes and political strategies.