This chapter introduces the status and challenges of technologically and commercially practical Mg-ion insertion cathodes. Research and development benefits greatly from investigating materials that do not meet technological goals in order to develop fundamental principles, mechanisms, or understanding of phenomenon not readily understood under more challenging conditions (i.e. high voltage Mg insertion processes). A breakthrough, however, in the elusive quest for transformative high energy density magnesium insertion cathodes would permanently alter the face of battery and electrochemical science. Techno-economic modelling and first-principles computational materials science are critical tools for designing and introducing transformative materials that can meet – or exceed – today's technology demands. Close-packed oxide materials are an obvious starting point for examination. Experimental evidence of magnesium extraction/insertion in two oxo-spinel systems is explored herein. Realizing the full potential of these systems will require significant future investment, such as understanding the hierarchy, relative role, and formation energetics of crystalline complexities, e.g. defects and site-ordering, on magnesium cation insertion, transport mechanisms, and stability of a wide array of materials classes. As will be seen, a lot of progress has been made to accomplish a next-generation insertion cathode and the framework for future work has been established.