The international landscape concerning ways in which defenses in patent litigation can be based on invalidity arguments has become ever more complicated in recent years. There are some countries in which patent validity must be argued in patent litigation procedures in the same case as the infringement action at ordinary civil courts. In other countries, validity must be made the subject of a separate procedure in the civil courts, i.e., in the form of mandatory bifurcation. There are also other countries which have mixed systems in which defendants have the choice of either arguing validity in litigation or in a separate procedure (e.g., utility model cases in Germany). Furthermore, there are some systems where the applicant has a choice between various routes to contest validity (e.g., opposition and invalidation procedures). And a further variation exists in countries like Germany, where, depending on the timing of the litigation, the use of post-grant opposition procedures in litigation may be permissive or even mandatory before invalidation procedures can be initiated at specialized courts, like the German Federal Patent Court. This variety of mechanisms to assert patent invalidity has led to a remarkable degree of forum-shopping, particularly in Europe. This panel will lead a discussion on the mechanisms and pros and cons of the various invalidity procedures available around the world.