The High Frequency Instrument (HFI) on Planck will obtain all-sky images of the cosmic microwave background and other astrophysical sources of emission with resolution of 9 arcmin at 100 GHz, 7 arcmin at 143 GHz and 5 arcmin at 217, 353, 545 and 857 GHz. The HFI focal plane will contain 48 silicon nitride, micromesh, bolometric detectors operating from a 100 mK heat sink. Four detectors in each of the 6 bands will detect the sum of the power in both linear polarizations. Additionally, four pairs of detectors will provide sensitivity to linear polarization of emission at 143, 217 and 353 GHz. We report on the development of these detectors, which are being produced at the JPL Micro Devices Laboratory, packaged at JPL Electronics Packaging, characterized at 100 mK before delivery to our HFI consortium partners at Cardiff University.