The European Court of Justice has played a crucial role in shaping how income tax systems in EU countries interact with each other. They have worked to balance the needs of the single market with the traditional principles of income taxation, such as worldwide income taxation and tax treaties. The Court has focused on ensuring fair treatment for individuals and companies while also considering the financial interests of EU Member States. Recent developments show a shift towards preventing tax avoidance and ensuring a balanced allocation of taxing rights between countries.