In this paper the question of tax assignment in multiple-level systems of government is addressed. Existing economic theory of fiscal federalism, which is the main source of information on tax assignment, is reviewed and various criticisms of this theory, as a normative theory, are outlined which indicate major difficulties in application to practice. Where a strong desire for decentralisation exists, such as in Spain, an alternative political-economic approach is suggested. This is then used to comment on developments in Spain where, it is concluded, the present assignment of taxing powers is insufficient to guarantee the autonomy of the regional governments. For Spain, regional access to a share of the VAT or personal income tax is suggested as the best option for tax reform in order to offer sufficient regional tax autonomy.