Microseismic monitoring is the only technique to provide 3D data to geotechnical engineers at underground mines and has become a standard tool in the deeper metalliferous and coal mines around the world. Objectives of the monitoring are typically rescue, prevention, control, warnings (which include identification of precursors for goafs and for large seismic events which may be associated with rock/coal-bursts) and back-analysis. Depending on the monitoring objectives, the required monitoring system sensitivity – the minimum moment magnitude above which seismic events are reliably recorded and quantified – would be between magnitude -2.0 and magnitude +1.0. While simple seismic activity – crack counting – is sometimes good enough for warnings of impending goaf occurance, precursors of large seismic events associated with rock-bursts or coal-bursts are more difficult to identify. There have been some sporadic successes in metalliferous mines with high levels of seismic activity – and thus lots of large seismic events to calibrate against. The design of the seismic sensor array depends on the objectives, but typically involves sensors installed in the main and tail gates either side of the panel. Current best practice involves permanent installation into long up- and down-holes to achieve the 3D configuration required for reliable 3D location of seismic sources. A more cost-effective solution is to use temporarily-installed geophones along with a surface seismic station.