Semiconductor radiation detectors play an important role in radiation instrumentation. They have many advantages due to their small size, outstanding energy resolution in nuclear spectroscopy, easy pixilation for high spatial resolution and ability of integration with readout electronics. Advancement in microelectronic industry made silicon one of the most popular material for radiation detectors in medicine with applications in diagnostic and cancer treatment. Radiation therapy is one of the part of radiation medicine associated with cancer treatment. Today almost half of cancer patients are treated by radiation. Clinical outcome of radiation therapy depends on accurate delivery of radiation to the tumour while sparing the normal tissue. Detectors for in vivo real time radiation dosimetry and understanding of radiobiological properties of radiation are crucial for improvement of clinical outcome of treatment. In this chapter presented state-of-the-art in development and applications of silicon semiconductor radiation detectors for quality assurance in radiation therapy including integral dosimetry in photon, electron and neutron therapies, new silicon detectors for microdosimetry in hadron therapy and mini-dosimetry for synchrotron radiation therapy. Applications in external beam therapy and brachytherapy have been covered. Amorphous silicon imaging detectors for portal imaging of medical LINACs and other solid state miniature radiation detectors like fiber optic dosimetry and OSL dosimetry for radiation therapy have been reviewed shortly. © 2006 Springer.