The performance of a network, such as a mobile communications network, having multiple network nodes can be monitored. In a first network node, certain data flows passing through the node are tagged by adding a performance management identifier. In a second network node, statistics relating to data flows containing the performance management identifier are aggregated, and the aggregated statistics are reported to a management node of the communications network. This allows for the gathering of accurate statistics relating to specific types of data flow, such as data flows to specific types of user equipment or containing specific types of data, even though the second node, in which the statistics are generated, cannot itself distinguish between the different data flows.