Microseismic monitoring has been used to image the growth behaviour of hydraulic fractures in the Barnett Shale, Texas1 . The Barnett is a naturally fractured shale reservoir, which causes significant variability and complexity in fracture growth during well stimulation operations. Over the past two years, several Barnett treatments have been successfully imaged using microseismicity recorded in realtime during the stimulations. In this paper, we provide examples illustrating how realtime microseismic monitoring can be used to map the complexity and variability of slurry interaction with pre-existing fracture sets during the treatment. In one example, the dynamic spatial and temporal behaviour in microseismicity during a staged treatment procedure was used to vary the hydraulic fracture geometry. The observations described in this paper suggest that realtime microseismic recording and analysis can be used to map hydraulic fracture variability in naturally fractured reservoirs, evaluate the influence of pre-existing fractures on stimulations, and change stimulation procedures in response to observed hydraulic fracture variability.