Michael D. Johnson, Kazunori Akiyama, Lindy Blackburn, Katherine L. Bouman, Avery E. Broderick, Vitor Cardoso, Rob P. Fender, Christian M. Fromm, Peter Galison, José L. Gómez, Daryl Haggard, Matthew L. Lister, Andrei P. Lobanov, Sera Markoff, Ramesh Narayan, Priyamvada Natarajan, Tiffany Nichols, Dominic W. Pesce, Ziri Younsi, Andrew Chael, Koushik Chatterjee, Ryan Chaves, Juliusz Doboszewski, Richard Dodson, Sheperd S. Doeleman, Jamee Elder, Garret Fitzpatrick, Kari Haworth, Janice Houston, Sara Issaoun, Yuri Y. Kovalev, Aviad Levis, Rocco Lico, Alexandru Marcoci, Niels C. M. Martens, Neil M. Nagar, Aaron Oppenheimer, Daniel C. M. Palumbo, Angelo Ricarte, María  J. Rioja, Freek Roelofs, Ann C. Thresher, Paul Tiede, Jonathan Weintroub, Maciek Wielgus

• The Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravityThe Event Horizon Telescope (EHT) has led to the first images of a supermassive black hole, revealing the central compact objects in the elliptical galaxy M87 and the Milky Way. Proposed upgrades to this array through the next-generation EHT (ngEHT) program would sharply improve the angular resolution, dynamic range, and temporal coverage of the existing EHT observations. These improvements will uniquely enable a wealth of transformative new discoveries related to black hole science, extending from event-horizon-scale studies of strong gravity to studies of explosive transients to the cosmological growth and influence of supermassive black holes. Here, we present the key science goals for the ngEHT and their associated instrument requirements, both of which have been formulated through a multi-year international effort involving hundreds of scientists worldwide.…