The dominance of converter-based generation in power systems results in a significant reduction of the number of conventional power plants. The transition introduces major challenges of substituting synchronous generators and their ancillary dynamic control services with converter-interfaced generations whose control and interaction with the grid have not been fully understood. This paper presents challenges and research opportunities of frequency control of such low inertia systems. Challenges of frequency control in converter-based systems and a review of power systems facing the challenges caused by low inertia conditions around the world are first investigated. Then, a summary of the solutions that have been proposed for frequency control in low inertia systems is analysed. The paper will conclude with research opportunities for frequency control in low inertia systems, which require further investigation for converter-interfaced systems.