The Network on Fiscal Relations has been assessing the degree of sub-central government tax autonomy in OECD countries for almost two decades. This paper provides an in-depth description of the methodology used to characterise tax autonomy. After summarizing the wide-spread use of the tax autonomy results by researchers addressing a range of policy issues, the paper highlights recent trends in sub-central government revenues and presents the results of the latest survey of tax autonomy, completed in 2017. Using the OECD’s tax autonomy methodology, the paper for the first time assesses local government tax autonomy in the 50 US states. The analysis reveals that US local governments have somewhat more tax autonomy than local governments in the average OECD country. The paper includes suggestions for further refinements of the tax autonomy methodology.