JUICE - JUpiter ICy moons Explorer - is the first large mission in the ESA Cosmic Vision programme. Due to launch in May 2022 and arrival at Jupiter in October 2029, it will make detailed observations of the Jovian system, with a special focus on the planet itself, its giant magnetosphere, and the three icy moons: Ganymede, Callisto and Europa. The current mission baseline assumes a Jupiter tour of almost three years including two close flybys of Europa, fifteen flybys of Ganymede, and twelve flybys of Callisto, together with a high inclination phase of six months in order to characterize Jupiter high latitudes. In August 2032, JUICE will then orbit Ganymede for at least ten months, down to an altitude of 500km. The first goal of JUICE is to to characterize the conditions that might have led to the emergence of habitable environments among the Jovian satellites. Ganymede is a high-priority target as it provides a unique laboratory for analyzing the nature, evolution and habitability of icy worlds, including the characteristics of subsurface oceans, and its unique magnetosphere interaction with the Jovian magnetodisc. For Europa, the focus will be on recently active zones, while Callisto will be explored as a witness of the early Solar System. JUICE will also investigate the Jupiter system as an archetype of gas giants. The circulation, meteorology, chemistry and structure of the Jovian atmosphere will be studied from the cloud tops to the thermosphere and ionosphere. JUICE will also study the properties of the magnetodisc, and will analyse the coupling processes within the magnetosphere, ionosphere and thermosphere. The payload consists of 10 instruments plus a ground-based experiment (PRIDE) to better constrain the S/C position. A remote sensing package includes imaging (JANUS) and spectral-imaging capabilities from the UV to the sub-mm wavelengths (UVS, MAJIS, SWI). A geophysical package consists of a laser altimeter (GALA) and a radar sounder (RIME) for exploring the moons, and a radio science experiment (3GM) to sound the atmospheres and to determine the gravity fields. The in-situ package comprises a suite to study plasma and neutral gas environments (PEP) with remote sensing capabilities via energetic neutrals, a magnetometer (J-MAG) and a radio and plasma wave instrument (RPWI)