This chapter examines the origin and development of the concepts of subsidiarity and multilevel governance. While subsidiarity has been used as a legal tool against an excessive centralisation of powers at the European Union (EU) level, though it is mostly used to understand how different government levels interact with each other on issues that they share competences and/or interests. P. Craig makes little distinction between the discussion of subsidiarity in the EU and the multilevel complexity of national federal systems. While some authors regard subsidiarity as being essentially a political principle, others argue that such Impact Assessments on the pre-legislative phase could be used, under certain terms, by the European Court of Justice to adjudicate on matters of subsidiarity. In their debate, Craig and G. Davies summarise the discussion between whether subsidiarity is a philosophical or a political principle or whether it is a legal principle that is grounded on the praxis.