This thesis presents a novel optical magnetometer for high magnetic fields along with the first explorations of its applications in MRI. The magnetometer works by continuously tracking a magnetic-fielddependent optical resonance in atomic cesium. The technique employs a combination of well-established methods comprising sideband spectroscopy, saturated absorption spectroscopy, and FM spectroscopy. A large focus of this work has been on engineering a robust solution that could be operated in a hospital environment. This has resulted in a novel — but still mature — technology, with specifications that compare favorably to conventional and commercially available methods for high-field magnetometry. In order to calibrate the magnetometer, the magnetic-field-dependence of the relevant optical cesium resonance has been characterized. NMR magnetometry on pure water has been used as the absolute reference. This has resulted in accurate measurements of two universal cesium coefficients, describing the field dependence, such that magnetometry can now be performed using this technique with an accuracy in the ppmrange. The magnetometer has been used to map out two MRI sequences in a 7 T scanner, and to detect temporal instabilities and spatial nonlinearities in the gradients. This work establishes the field of optical magnetometry for MRI, with future possibilities including image corrections in e.g. error-prone sequences or gradient coil systems with relaxed technical requirements.