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 x 25 pixels, each, and two filled Si bolometer arrays with 16 x 32 and 32 x 64 pixels, respectively, to perform imaging line spectroscopy and imaging photometry in the 57-210 micron wavelength band. In photometry mode, it will simultaneously image two bands, 60-85 micron or 85-130 micron and 130-210 micron, over a field of view of ~ 1.75'x3.5', with full beam sampling in each band. In spectroscopy mode, it will image a field of ~ 50"x50", resolved into 5 x 5 pixels, with an instantaneous spectral coverage of ~ 1500 km/s and a spectral resolution of ~ 75 - 300 km/s. In both modes background-noise limited peformance is expected, with sensitivities (5σ in 1h) of ~3 mJy or 3-10x10-18 W/m2, respectively.