Many-body localization is closely connected to some fundamental questions of quantum mechanics, like how and why quantum systems thermalize. It can protect quantum order at elevated temperatures and can potentially be important in the development of quantum memories. Many-body localization occurs in isolated quantum systems when Anderson localization persists in the presence of finite interactions. Despite strong evidence for the existence of a many-body localization transition a reliable extraction of the critical disorder strength has been difficult due to a large drift with system size in the studied quantities. In this talk I describe the challenges involved in this problem and explain our approaches, based on entanglement entropy, to understand it: (i) the variance of the half-chain entanglement entropy of exact eigenstates and (ii) the long time change in entanglement after a local quench from an exact eigenstate. With this we can estimate the critical disorder strength and its energy dependence. We investigate these quantities in a disordered quantum Ising chain that also has disorder protected quantum order at large disorder strength and provide evidence for it being a separate transition.