FIR photon detector development starting from the extrinsic detectors for LWIR to FIR wavelengths are presented. Several other types of IR detectors, including the cut-off wavelength extension into the FIR range for quantum well infrared photodetectors (QWIPs), are summarized. Efforts in developing p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared detectors and the most reason developments on GaAs/AlGaAs Heterojunction interfacial workfunction internal photoemission (HEIWIP) far-infrared detectors are presented.