The present invention relates to environmental control systems. More particularly, the present invention relates to methods and apparatus for monitoring the performance of environmental control systems arranged to control at least one environmental characteristic in an environment. The method comprises: identifying an activation period during which the environmental control system is expected to be activated to control an environmental characteristic; recording a data set of sensor data measured during the activation period relating to the environmental characteristic; receiving control data indicating a target value configured at the environmental control system for the environmental characteristic; analysing the sensor data set in dependence on the target value to determine whether the sensor data is consistent with an expected response of the environmental characteristic during the activation period; and detecting an underperformance condition in response to determining that the sensor data is not consistent with the expected response.