Optical Packet Switching (OPS) is among the most promising solutions for next generation metropolitan networks. The increase of packet-based services (video on demand, etc.) is pushing metropolitan networks providers to renew their infrastructures. Today, metropolitan networks are based on SONET/SDH circuit-switched networks, which are becoming inefficient and costly to support new requirements of quality of service and bandwidth of sporadic packet-based traffic. To solve this problem, many new network solutions are proposed recently, including Next Generation SONET/SDH, Resilient Packet Ring, etc. Among others, the optical networking technology appears a good choice thanks to its following benefits: huge transmission capacity, high reliability, and high availability. This paper is devoted to provide an overview of the metropolitan network infrastructure and particularly to its evolution towards OPS networks. It also highlights performance issues in terms of optical packet format, medium access control protocol and quality of service, as well as traffic engineering issues