Increasingly, Sweden and other countries are making their taxes conditional. Companies are not allowed to deduct costs and losses if these can also be deducted in other countries, and income that would otherwise be tax-free must still be taxed if it is not taxed in another country. In order to counteract aggressive tax planning that gives rise to tax advantages, conditional legislation is now being introduced, followed by directives and recommendations stemming from international cooperation such as the EU, OECD and G20. At the same time, both national and EU law practice indicates that conditional legislation faces significant problems. EU Member States find it difficult to design legislation that is both fit for purpose and compatible with EU law. For taxpayers, conditional legislation proves to lead to double taxation and liquidity disadvantages. Furthermore, it gives rise to a lack of predictability, as its application requires knowledge of other states' legal systems, such as their tax bases, legal definitions and classifications, etc. Conditional legislation can therefore be seen as a way for political actors to shift problems resulting from the lack of harmonization of states' tax systems onto taxpayers and law enforcement authorities. The project aims to establish principles for the design and application of conditional legislation so that it achieves its objectives in ways that are compatible with EU law, but where its negative effects are minimized.