PIONIER conceptual scheme. The Injection and Optical Delay Unit (IOPDU) is responsible for injecting the four beams (either from CAU or from VLTI) in the single mode fibers. It also implements the OPD modulation, the tip-tilt correction, and the polarization control. The Integrated Optics Beam Combiner (IOBC) recombines the four beams into 24 interferometric outputs in a pairwise scheme. These outputs are then imaged, and eventually spectrally dispersed, by the Imaging Optics and Dispersion Unit (IMODU) on the detector (DET). The aim of the Calibration and Alignment Unit (CAU) is to provide four interferometric beams that match the size and angle of the VLTI beams.