The paper reviews the state of the art of cooling systems employed in electric traction motors. In recent years, the pursuit of sustainable and ecofriendly mobility solutions has pushed the research towards the development of electric vehicle powertrains. Permanent magnet electric machines are preferred over the other possible motors due to high efficiency and torque density. However, the performance of PM machines is deteriorated at high temperatures since the magnet remanence reduces with increasing temperature until a maximum allowable point, above which demagnetization occurs. Moreover, the reduction of remanence requires higher electric current to produce the demanded output torque which increases the winding loss significantly. The temperature rise of stator winding increases its DC resistance as well. In order to mitigate these problems, different solutions for the thermal problem have been researched and adopted, each one with its own pros and cons. In this paper, various cooling solutions of the electric machines, discussed in the literature, are compared qualitatively. Two main heat transfer mechanisms, conduction and convection, can be observed in the compared solutions. The conduction mechanism is effectively utilized between stator and motor housing using thermal conductive pads or resins. Under the convection heat transfer mechanism, different cooling solutions will be discussed such as air cooling, outer water jacket for stator cooling, water cooling for rotor, oil injection and oil spraying for stator and rotor cooling. The presented comparative analysis can be used to evaluate the pros and cons of various cooling systems and select an appropriate solution based on the constraints of the traction motors.