For most scientists, terraforming still lies solidly in the realm of science fiction. However, recent developments in climate, biotech, and space science demand reassessing its feasibility. To do so, we hosted a two-day workshop to formulate an up-to-date perspective on the feasibility of terraforming Mars. Attendees generally agreed that warming Mars and greening it with a photosynthetic biosphere are feasible and could be accomplished in relatively rapid timescales, perhaps less than a century. A green Mars would be conducive to agriculture and domed human settlements. Over much longer timescales, a green Mars could develop a planet-wide pure oxygen atmosphere, protecting the surface from harmful radiation and enabling human life ‘outside’ with no need for a magnetic field. These workshop proceedings summarize these discussions, highlight outstanding technical questions, and identify research priorities for the field moving forward.