The first workshop on Efficient Neutron Sources, ENS2019, was held at the Paul Scherrer Institut (PSI) in Villigen, Switzerland, on 2-5 September 2019. The workshop, organized by PSI and ESS (European Spallation Source), was part of the Work Package 4: Efficient Energy Management of the ARIES program. Neutron sources are used in various research fields, including condensed matter, fundamental physics, and medical applications. The workshop was dedicated to discuss the state-of-the-art and new ideas to design more efficient neutron sources. These proceedings consist of an introductory article summarizing the key points from the talks and from discussion sessions on the topical areas, followed by extended abstracts of the individual talks held by the participants.