  The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for
High-ENergy Astrophysics (Athena) will provide spatially resolved
high-resolution X-ray spectroscopy from 0.2 to 12 keV, with 5 arc second pixels
over a field of view of 5 arc minute equivalent diameter and a spectral
resolution of 2.5 eV up to 7 keV. In this paper, we first review the core
scientific objectives of Athena, driving the main performance parameters of the
X-IFU, namely the spectral resolution, the field of view, the effective area,
the count rate capabilities, the instrumental background. We also illustrate
the breakthrough potential of the X-IFU for some observatory science goals.
Then we briefly describe the X-IFU design as defined at the time of the mission
consolidation review concluded in May 2016, and report on its predicted
performance. Finally, we discuss some options to improve the instrument
performance while not increasing its complexity and resource demands (e.g.
count rate capability, spectral resolution).
  The X-IFU will be provided by an international consortium led by France, The
Netherlands and Italy, with further ESA member state contributions from
Belgium, Finland, Germany, Poland, Spain, Switzerland and two international
partners from the United States and Japan.
