The X-ray Integral Field Unit (X-IFU) on board the Advanced Telescope for High Energy Astrophysics (Athena), will provide high spectral resolution (2.5 eV up to 7 keV) over a 5 arc minute (equivalent diameter) field of view. The X-IFU is currently in the middle of its Phase A study phase. In this paper, we review the technical challenges (system level issues, TES array, readout, cooling, …) as identified during the first year of the study. This instrument is developed by a large international consortium under a French leadership. The Netherlands and Italy, as Co-PIs, with ESA member state from Belgium, Finland, Germany, Poland, Spain, Switzerland, are the European contributors, with additional contributions from the United States and Japan.