Methods and systems for deriving measures indicative of application-level quality of service in relation to a client application operating in a data network whereby application data of relevance to the client application is transmitted in packets from node to node over the network by means of a plurality of application flows, each application flow being an exchange of dat between the client application and a serving application; the method comprising: monitoring characteristics of packets containing application data of relevance to the client application and identifying an application flow associated with each packet; determining from monitored characteristics, according to predetermined criteria and individually in relation to each of a plurality of application flows associated with the client application, a flow-level performance measure; and collating the respective flow-level performance measures and deriving an application-level service-quality measure.