Most brachytherapy procedures utilize radioactive materials. The application and use of these materials necessitate robust safety measures to protect patients, staff, and the general public, as well as calibration of sources for clinical use. Gas-filled radiation detectors, particularly the ionization survey meter and Geiger–Mueller (GM) detector, are essential instruments for any brachytherapy practice. This chapter provides an overview of the theory, operation, and calibration of these devices. Additionally, the role of gas-filled detectors in radioactive material-based brachytherapy procedures, including common problems and emergency procedures, will be reviewed.