Monitoring is an important concept in network management as it helps network operators to determine the behavior of a network and the status of its components. Traffic engineering, quality of service, and anomaly detection also depend on monitoring for decision making. Software-defined networking (SDN) is becoming increasingly popular for network provision and management tasks. This paper surveys the tasks and challenges associated with SDN, providing an overview of SDN monitoring developments. Several design concepts, research directions, and open issues are also discussed.