The aim of the paper is to discuss the challenges in the competition assessment of 5G mobile network sharing which emerge in addition to those which were relevant under 4G networks (or below). Chapter 2 of the paper briefly discusses the main types of mobile network sharing. Chapter 3 presents our analytical framework for the competition assessment of mobile network sharing agreements, built on the approach laid out in guidelines by the European Commission and European regulators, as well as competition cases in European jurisdictions. Throughout, we focus on radio access network (RAN) sharing. Chapter 4 brings 5G into the picture. Here we discuss five special 5G technology and service characteristics which pose new challenges to the competition assessment of 5G RAN sharing agreements and should be in the focus of future research.