The last decade has seen a revolution in our cosmic census of stellar-mass black holes (BHs). BH mergers at cosmological distances are now routinely detected through gravitational waves. In the Milky Way, BHs have been detected both in isolation and in a diverse sample of binary systems spanning four orders of magnitude in orbital period. Ongoing and planned observational efforts will dramatically expand the BH census. Despite the progress on the observational front, basic questions about stellar-mass BHs remain unanswered. BH mergers are rare outcomes of binary evolution. A diverse zoo of evolutionary models can all match the observed BH merger rate, but none can explain all the properties of the observed population. No models satisfactorily explain the formation of most of the known local BHs in binaries, and the population of isolated BHs is almost completely uncharted.

This program will bring together theorists and observers to understand and exploit the new data. Some of the questions to be addressed include: What are gravitational wave detections telling us about the life, death, and afterlife of massive stars? What constraints do the new surveys place on the key uncertainties in binary evolution and BH formation? Which predictions for binary evolution and final fate are robust? Do dynamical formation channels predict BH binary populations consistent with observed systems? How can we use time-domain astrometry and radial velocity surveys efficiently? What can be inferred about BH natal kicks? How do we prepare for the flood of microlensing events from the Rubin Telescope?