The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments for ESA's far infrared and submillimetre observatory, Herschel. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16×25 pixels, each, and two filled Si bolometer arrays with 16×32 and 32×64 pixels, respectively, to perform imagiung line spectroscopy and imaging photometry in the 57-210μm wavelength band. In photometry mode, it will simultaneously image two bands, 60-85μm or 85-130μm and 130-210μm, over a field of view of ~1.75'×3.5', with full beam sampling in each band. In spectroscopy mode, it will image a field of ~50"×50", resolved into 5×5 pixels, with an instantaneous spectral coverage of ~600-2500 km/s and a spectral resolution of ~75-300 km/s. In both modes background-noise limited performance is expected, with sensitivities (5σ in 1h) of ~3 mJy or 3-20×10-18W/m2, respectively.