The multi-unit spectroscopic explorer (MUSE) is an integral field spectrograph installed at the Very Large Telescope (VLT) of the European Southern Observatory (ESO). It operates in the visible wavelength range, and combines a wide field of view with a fine spatial sampling and a large simultaneous spectral range. It is designed to take advantage of the improved spatial resolution provided by adaptive optics. MUSE had first light on the VLT on 31 January 2014. Background Traditionally astronomical observations in the optical region have been separated into imaging and spectroscopy. The former can cover a wide field of view, but at the cost of a very coarse resolution in the wavelength direction. The latter has tended to either lose spatial resolution - completely in the case of fibre spectrographs, and partially in the case of long-slit spectrographs - or to have only coarse spatial resolving power in the case of recent integral field spectrographs. MUSE was devised to improve on this situation by providing both high spatial resolution as well as a good spectral coverage. The principal investigator of the instrument is Roland Bacon at the Lyon Centre for Astrophysics Research (CRAL) in charge of a consortium consisting of six major European institutes: CRAL at Lyon Observatory is the PI institute and led the construction of the majority of the instrument. Other involved institutes include the German Institut für Astrophysik Göttingen (IAG) and the Leibniz Institute for Astrophysics Potsdam (AIP), the Netherlands Research School for Astronomy (NOVA), the Institut de Recherche en Astrophysique et Planétologie (IRAP), France, ETH Zürich, Switzerland as well as the European Southern Observatory (ESO). The kick-off for the project was January 18, 2005 with the final design review in March 2009. The instrument passed its final acceptance in Europe on September 10, 2013 MUSE was mounted on the Nasmyth platform of the fourth VLT Unit telescope on January 19, 2014, and saw first light on January 31, 2014. Scientific goals Stars and resolved stellar populations MUSE has a field of view that is well-matched to a number of fascinating objects in the Milky Way, such as globular clusters and planetary nebulae. The high spatial resolution and sampling will enable MUSE to simultaneously observe the spectra of thousands of stars in one shot in dense regions such as globular clusters. In star-forming regions, with a mixture of ionised gas and stars, MUSE will provide information both on the