Carbon capture and storage (CCS) is currently one of several candidate technologies for reducing the emission of industrial CO2 to the atmosphere. As plans for large-scale geological storage of CO2 are being considered, it is clear that monitoring programs will be required to demonstrate security of the CO2 within the storage complex. Numerous geophysical monitoring techniques are currently being tested for this purpose, including controlled-source time-lapse reflection seismology, satellite synthetic aperture radar interferometry, electromagnetic sounding, gravity, and others. Passive seismic monitoring is an additional technique under consideration that complements these other techniques, and has potential as a cost-effective method of demonstrating storage security. This is particularly true over longer periods of time, as passive seismic arrays cost relatively little to maintain. Of the large-scale CCS pilot projects currently operational, thus far only the IEA GHG Weyburn-Midale CO2 Monitoring and Storage Project has included passive seismic monitoring. Here we present the results from five years of passive seismic monitoring at Weyburn, and discuss the lessons learnt that can be applied when deploying passive seismics to monitor future CCS operations.