The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments for ESA's far infrared and submillimetre observatory Herschel. In its photometry mode, it will simultaneously image two bands, 60-85 or 85-130μ m and 130-210μ m, 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 ˜ 1500km/s and a spectral resolution of ˜ 175km/s. In both modes near-background-noise limited peformance is expected, with sensitivities (5 σ in 1h) of ˜ 4mJy or 3-20× 10-18 W/m2, respectively. We describe the observing modes of the instrument and illustrate the scientific potential of PACS with examples from the emerging Guaranteed Time Programmme of the PACS consortium. This programme presently includes extragalactic photometric surveys to identify the constitutents of the Cosmic Infrared Background, detailed studies of individual high-z objects, star formation and activity in nearby, infrared-bright galaxies, the physics of the ISM in low metallicity galaxies, surveys and pointed observations of different phases of star formation, and circumstellar environments and the late phases of stellar evolution. This work is supported by the following funding agencies: ASI (Italy), BMVIT (Austria), CEA/CNES (France), DLR (Germany), ESA-PRODEX (Belgium), and CDTI (Spain).