An important recent development has been the exploitation of the seismic noise field by the use of correlations between seismograms recorded at different positions. We discuss the nature of the ambient noise field, which is dominated by microseismic signals generated in the oceans. The dominant component of the correlation field comes from surface waves, and cross-correlation procedures can extract an empirical Green’s function representing propagation along the path between the stations being correlated.Such results are widely exploited in ambient noise tomography. In some circumstances body waves can also be extracted from the noise field.