This forum compared the different monitoring techniques in order to better understand the interpretability of their results. The recent increase in awareness of the potential of unconventional reservoirs has caused a tremendous growth in hydraulic fracture stimulation operations. Conventionally, the recording of microseismicity has been performed with borehole seismic tools deployed in deep nearby wells but "frac monitoring" is not restricted to such a setting. Elastic waves generated by these micro-earthquakes can also be recorded at the earth's surface and in shallow boreholes. The energy released by microseismic events as a result of hydraulic stimulation is small, and, due to the longer travel paths and higher noise levels, monitoring hydraulic fracture stimulations from surface potentially requires orders of magnitude more sensors than downhole monitoring.