  This paper provides an overview of the Low Frequency Instrument (LFI)
programme within the ESA Planck mission. The LFI instrument has been developed
to produce high precision maps of the microwave sky at frequencies in the range
27-77 GHz, below the peak of the cosmic microwave background (CMB) radiation
spectrum. The scientific goals are described, ranging from fundamental
cosmology to Galactic and extragalactic astrophysics. The instrument design and
development are outlined, together with the model philosophy and testing
strategy. The instrument is presented in the context of the Planck mission. The
LFI approach to ground and inflight calibration is described. We also describe
the LFI ground segment. We present the results of a number of tests
demonstrating the capability of the LFI data processing centre (DPC) to
properly reduce and analyse LFI flight data, from telemetry information to
calibrated and cleaned time ordered data, sky maps at each frequency (in
temperature and polarization), component emission maps (CMB and diffuse
foregrounds), catalogs for various classes of sources (the Early Release
Compact Source Catalogue and the Final Compact Source Catalogue). The
organization of the LFI consortium is briefly presented as well as the role of
the core team in data analysis and scientific exploitation. All tests carried
out on the LFI flight model demonstrate the excellent performance of the
instrument and its various subunits. The data analysis pipeline has been tested
and its main steps verified. In the first three months after launch, the
commissioning, calibration, performance, and verification phases will be
completed, after which Planck will begin its operational life, in which LFI
will have an integral part.
