Ultra-diffuse galaxies (UDGs) were recently recognized as an abundant class of low-surface brightness galaxies with unusually large sizes -- found both in galaxy clusters and in the field. The nature and origins of these galaxies are unclear, with one intriguing possibility that some of them are "failed Milky Ways" with massive halos but a paucity of stars. I will present observations of stars and globular clusters in UDGs that constrain their stellar populations and masses -- including evidence for being ultra-rich in dark matter. I will also show results from simulations of UDG formation through ram-pressure stripping of gas-rich disk galaxies.