A gear is an essential component of most mechanical or electromechanical systems. Mechanical gears are readily used in automobile and industrial applications for torque transmission and speed variation purposes. Recent significant increase in research and development activities on magnetic gears (MG) indicates its potential in effectively replacing mechanical gears from industrial and other applications. Thus, the gradual developments of MG and magnetically geared machines have opened up innumerable opportunities for engineers and researchers. However, being a relatively new technology, the concept of MG is still unclear to many potential researchers. This paper is aimed at presenting an overview of the state of the art in MG technology, the gradual development of MG topologies, its design characteristics, optimization techniques, and a guideline for selecting suitable materials for gear construction. The MG technology is still in its early stage of development but has a huge opportunity for further improvement. The MG not only promises an ability to replace the mechanical gear, but also offers many other added advantages including increased lifespan of the machine, maintenance cost reduction, and reliable operation. Although intensive research works are being done on MGs, it is presently still unable to replace mechanical gears in industrial applications due to the holdback from the industry-readiness of MGs. The limitations and future possibilities discussed at the end of this paper could benefit readers interested in developing innovative ideas for an industry-ready version of MG.