Imaging techniques have played a major role in the diagnosis, treatment and management of breast cancer. A signicant drop in mortality rates among breast cancer patients was observed over the past decade and this has been attributed primarily to widespread screening programs using X-ray mammography. As we advance our knowledge of this disease and move towards more tailored treatment options, it has become apparent that there is a need for more advanced and dedicated imaging techniques that will allow for more thorough evaluation of the disease non-invasively. To address this need, a simultaneous PET-MRI breast scanner was built. Positron emission tomography (PET) can provide functional or biological information of the tumor under study. With advances in radiotracer chemistry, information that was only available after biopsy and evaluation of tumor tissue, can now be obtained non-invasively using PET imaging. The addition of Magnetic Resonance Imaging (MRI), an imaging modality that provides high-resolution structural image, will result in an imaging modality that combines the best of both worlds- anatomy and physiology. Hence, such a device will allow earlier diagnosis, and better patient care, along with being an important research tool. The simultaneous PET-MRI system consists of a PET ring that is MRI compatible. The PET ring has a transaxial imaging eld of view of 108 mm and axial height of 18 mm. The ring is mounted on a translation device that allows for greater area coverage as necessary. The PET ring is positioned within the radio frequency coil of a dedicated breast MRI system, onto which the patient is positioned in the prone position. The breast tissue as a result is in the eld of view of both the PET and MRI systems, thereby facilitating simultaneous PET-MRI imaging. The system was tested and optimized to minimize inter-modality interference and simultaneous PET-MRI studies were conducted on breast cancer patients. The development of this device and its performance evaluation, both on the bench and in the clinical setting is the work of this thesis.