The use of preclinical imaging in research spans several decades and developments in the field are continuing in a number of directions, with research using simultaneous hybrid magnetic resonance-positron emission tomography (MR-PET) systems at the forefront. This opens up new opportunities to access large amounts of diverse temporal and spatial information with high accuracy. Following on from the human MR-PET instrumentation chapter, this chapter deals with the particular challenges, such as ultra-high magnetic and gradient field strengths associated with the combination of preclinical MR and PET hardware. Furthermore, the development of several small animal systems for combined MR-PET applications are also introduced.