  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 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.
