We describe the development of sensitive extrinsic germanium focal plane arrays for the Space Infrared Telescope Facility (SIRTF). The cryogenic telescope of SIRTF will be used for natural-background-limited astronomical observations, an application that requires arrays of far-infrared sensors of unprecedented sensitivity. For the 30 - 55 μm wavelength range, we describe the development of monolithic Ge:Be arrays with associated readouts bump- bonded to a common substrate. For the 55 - 120 μm wavelength range, we describe arrays based on a 1 × 32 building block that can be stacked into fully filled two- dimensional arrays. We show results from tests of a 3 × 32 demonstration array that operates at the SIRTF performance levels. This array demonstrates solutions to the problems of (1) maintaining the required detector operating temperature (< 2 K); (2) isolating the array from the heat and photon emission of the readout; (3) achieving long absorption paths in the detectors; and (4) maintaining constant detector bias and low noise from the readout amplifier. The array design permits stacking into the full 32 × 32 format required for SIRTF. The array thermally isolates the readout from the detectors to take advantage of the improved electronics noise performance at the elevated temperature of 20 K. We also show designs for improved versions of the array that emphasize manufacturability and the ability to be space qualified.