The Cosmic Ultraviolet Baryon Survey (CUBS) program is a large Hubble Space Telescope Cycle 25 General Observer program (GO-CUBS; PID=15163). It is designed to map 'dark' baryonic structures in the crucial but unconstrained epoch between redshift z = 0.4 and z ~ 2, by combining absorption-line spectroscopy of 15 UV-bright QSOs at z > 0.8 with matching deep galaxy survey data. This legacy sample will enable systematic studies of the co-evolution of galaxies and CGM/IGM at a time when the star formation rate density undergoes its most dramatic changes, and provide key insights into how galaxy growth is regulated by accretion and outflows.