Planck is designed to image the anisotropies of the Cosmic MicrowaveBackground (CMB) over the whole sky, with unprecedented sensitivity (ΔT/T ~ 2 × 10-6) and angular resolution (~5 arcmin). Planckwill provide a major source of information relevant to several cosmologicaland astrophysical issues, such as testing theories of the early universe andthe origin of cosmic structure. The ability to measure to high accuracy theangular power spectrum of the CMB fluctuations will allow the determination offundamental cosmological parameters with an uncertainty of order a fewpercent. In addition to the main cosmological goals of the mission, the Plancksky survey will be used to study in detail the very sources of emission which“contaminate” the signal due to the CMB. This will result in a wealth ofinformation on the properties of extragalactic sources, and on the dust andgas in our own galaxy. One specific notable result will be the measurement ofthe polarized emission from our own Galaxy. The ability of Planck to measurepolarization across a wide frequency range (30–350 GHz), with high precisionand accuracy, and over the whole sky, will provide unique insight into theproperties of the interstellar medium. An overview is presented of the Planckmission, its scientific objectives, the key elements of its technical design,and its current status.