 We will briefly sketch each of them, along with their pros and cons. Chakraborty and Lochan (2017), Bryan and Medved (2017), Marolf (2017), and Unruh and Wald (2017) provide recent reviews of the most popular approaches, with Marolf's emphasizing possible approaches that save unitarity, and Unruh and Wald's emphasizing ones that violate it. (See Mathur 2009 and Chen et al. 2015 for recent discussions of approaches based on remnants, which we will not cover here.) 