Constitutional review entails checking the compatibility of laws and other acts with the constitution. This can be centralised in the hands of a single specialised body outside the regular judiciary, and most EU member states have chosen such a system. Alternatively, such a review can be diffused, so that all courts from the lowest to the highest carry this out. Under such a system, each judge can apply the constitution, and thus not apply a law that conflicts with the constitution. In the EU, for example, Denmark, Ireland, Estonia and Sweden use this system.

Typically, three different actors can bring cases to a constitutional court: political actors, ordinary judges, and individuals.

The review of constitutionality can be strong or weak. Strong review courts have the last word (as opposed to the parliament), in the sense that their interpretations are binding on political branches of government. They can invalidate legislation. With a weak review, the legislature and executive can reject court rulings. In such cases, courts are first given an opportunity to explain why a challenged law is unconstitutional. Having done so, the legislature can then respond. The legislative deliberations are thus informed, but not bound by the court’s arguments. Nevertheless, in most cases, mild review allows the courts to invalidate legislation that is in clear conflict with the constitution.

The review can be abstract (detached from any particular application to the facts of a case) or concrete (dealing with a specific case in which the constitutional question is raised).