Since 30 years, European rules have significantly impacted national taxes on the income and property of individuals and businesses. However, not legislation adopted in Brussels, but the decisions of the European Court of Justice at times reduce the national budgets of the 27 EU Member States by several billions of Euros. This dissertation analyses, using network science, the system of rules that the Court has developed. Although some regard this form of European control as an undemocratic attack on national sovereignty, this research finds that the Court consistently recognises the legitimate right of Member States to levy taxes in their national territory.