The present invention relates to a method of quality of service monitoring of at least one service in a communication network. The method comprises a first and a second modes. The first mode comprises detecting degradation in quality of service in a communication network by selecting (80) a set of terminal service sessions for providing terminal service session reports; collecting (82) terminal service session information from the selected terminal service sessions; and determining (84) the quality of service in the communication network from the terminal service sessions monitored in the first mode. The second mode, being entered when a degradation of service quality in at least part of terminal service sessions is detected (86), comprises identifying at least one factor causing degradation of service quality in the communication network in steps of determining (88) at least one potential factor associated with terminal service sessions having degraded service quality as a candidate factor potentially causing the observed service quality degradation in at least part of the terminal service sessions; collecting (90) terminal service session information from terminal service sessions associated with at least one candidate factor; and evaluating (92) collected terminal service session information for service sessions associated with a candidate factor to identify whether the candidate factor is a cause of service quality degradation.