Ramp metering is one of the most important measures to alleviate congestion for urban expressway traffic. For comprehensive understanding of the cutting-edge research on ramp metering and exploring its development trend, this paper reviewed the development of ramp metering over the last 60 years since it was initially proposed, summarized the control logic and classification of ramp metering, and focused on the evolution of ramp metering control methodology. Research has found that: the control scope of ramp metering develops from local optimum to global optimum; the control object of ramp metering develops from macroscopic traffic flow control to microscopic vehicle control; the model formulation of ramp metering evolves from deterministic, homogenous and static modeling with strong assumptions to stochastic, heterogenous and dynamic modeling; the solution methods of ramp metering include mathematical derivation and search algorithms, which have advantages in terms of convergence and applicability, respectively. Finally, this paper analyzed current researches on ramp metering and recommended four future research directions, including coordinated macroscopic and microscopic traffic flow control, robust and resilient ramp metering, efficient and reliable solution to large-scale control problems, and implementation of intelligent control technology in connected environments.