The advent of hybrid MR-linear accelerators (MR-Linacs) has allowed the use of MRI-guidance to align the patient with respect to the radiation beam, track the tumor motion during treatment, and adapt the treatment plan based on the daily anatomy. To characterize MR-Linacs for patient treatments, specialized radiation detectors are needed since the presence of an external magnetic field impacts the response of the detector. Reference and relative dosimetry are impacted differently, and the severity of the impact is dependent on the detector. In this chapter, we have described the unique ways the magnetic field can impact various detector types including ionization chambers, scintillators, solid-state detectors such as diodes/diamonds/thermoluminescent detectors (TLDs)/optically-stimulated luminescence detectors (OSLDs), and chemical dosimeters such as radiochromic films.