the dynamical structure of these objects. Combining this with environmental information due to the wide field of view (1 arcminute corresponds to 430 kilo-parsec at a redshift of 0.7) it will be possible to study how the properties of galaxies are affected by the environment they find themselves in a very powerful, and mostly new, way. Science with the narrow field mode MUSE will also have a high spatial resolution mode with a field of view of 7.5x7.5 arcsec and a spatial resolution of 0.042 arcsec at 750 nm. The main scientific use of this mode is for studying in detail more nearby systems such as the environment around supermassive black holes in nearby galaxies. In particular it will be possible to resolve the sphere of influence of the black holes in most massive galaxies out to the Virgo cluster and for the most massive galaxies also in the Coma cluster of galaxies. Closer to home, MUSE will be able to study jets in nearby star forming regions and the surfaces of a range of solar system objects. This could for instance be used to carry out spectroscopic monitoring studies of volcanic activity on Io and spectroscopic studies of the atmosphere of Titan. Technical To meet the scientific aims of the instrument, MUSE has had to fulfill a number of requirements: The instrument should have high throughput. The ability to carry out very long integrations, thus the instrument has to be very stable. In combination with adaptive optics the instrument should achieve allow a boost in spatial resolution relative to seeing limited observations across the field of view across the celestial sphere. A wide field of view, allowing survey operation Efficient production to reduce cost, and efficient design to fit within the volume and mass constraints. To achieve the latter two points, the spectrograph consists of 24 identical integral field units (IFU), hence reducing cost by replication. These have each excellent image quality and light in the instrument plan is slice up and sent to individual IFUs using an image slicer. The spectrograph design has achieved an excellent image quality across the spectral bandwidth of MUSE with the tilt of the detector compensating for axial chromaticism. With such a design, expensive optical materials such as CaF2 are not needed, thus reducing the overall cost. The throughput is kept high by using high quantum efficiency CCDs. There is also only one