Since the theoretical prediction of room temperature intrinsic ferromagnetism in doped ZnO, there has been a huge amount of research in the area. Such a property would make ZnO an excellent candidate for spintronic materials, which would potentially have many advantages over the technologies currently available. Whilst some see room temperature intrinsic ferromagnetism upon doping ZnO with small amounts of transition metals, others present evidence of ferromagnetism from extrinsic transition metal clusters or have failed to see ferromagnetism at all. This thesis investigates the properties of doped ZnO thin films in an attempt to increase the understanding of this material.