This paper surveys the nexus between tax competition and the fiscal constitution. We distinguish various approaches to competition between jurisdictions and provide a critique of evolutionary approaches to 'systems competition'. In the main part of the paper, a simple model of horizontal tax competition, based on the Zodrow-Mieszkowski- approach, is developed that can be extended to address 'reasons for rules' in a tax competition context. Specifically, we discuss the internalisation of fiscal externalities in a federation and the timing of tax policy decisions, extending the existing literature in the latter regard. Several tentative constitutional recommendations are derived.