Enforceability of patent rights is the backbone of the patent system. We review differences in the way patent litigation systems are designed across jurisdictions. We also discuss challenges in collecting and accessing patent litigation data as well as their economic analysis. We provide some descriptive analysis of patent litigation in the U.S. and UK for the period 2010-2016 and 2007-2013, respectively. We also analyze administrative post-grant validity challenges in form of the inter partes review in the U.S. and oppositions at the EPO.