  In this paper we present the Low Frequency Instrument (LFI), designed and
developed as part of the Planck space mission, the ESA program dedicated to
precision imaging of the cosmic microwave background (CMB). Planck-LFI will
observe the full sky in intensity and polarisation in three frequency bands
centred at 30, 44 and 70 GHz, while higher frequencies (100-850 GHz) will be
covered by the HFI instrument. The LFI is an array of microwave radiometers
based on state-of-the-art Indium Phosphide cryogenic HEMT amplifiers
implemented in a differential system using blackbody loads as reference
signals. The front-end is cooled to 20K for optimal sensitivity and the
reference loads are cooled to 4K to minimise low frequency noise. We provide an
overview of the LFI, discuss the leading scientific requirements and describe
the design solutions adopted for the various hardware subsystems. The main
drivers of the radiometric, optical and thermal design are discussed, including
the stringent requirements on sensitivity, stability, and rejection of
systematic effects. Further details on the key instrument units and the results
of ground calibration are provided in a set of companion papers.
