Inertia plays an essential role in maintaining the frequency stability of power systems. Nevertheless, the increase of power electronics-based renewable generation can dramatically reduce the inertia of modern power systems. This issue has already challenged the control and stability of small-scale power systems. It will soon be faced by larger power systems as the trend of renewable integration continues. In view of the inertia challenge, this paper presents a comprehensive review of conventional and emerging inertia enhancement techniques. It is revealed that the inertia emulation by wind turbines has successfully demonstrated its effectiveness and will receive widespread adoptions. In addition, the virtual inertia generated by the DC-link capacitors of power converters has a great potential due to its low cost. The same concept can also be applied to ultracapacitors. Moreover, batteries will serve as an alternative inertia supplier. In future power systems where most of the generators and loads are connected via power electronics, virtual synchronous machines (VSMs) will become the main inertia provider. In general, it is concluded that advances in semiconductors and control promise to make power electronics an enabling technology for inertia control in future power systems.