The autocorrelation of the signals at a seismic station corresponds to the scenario with a coincident virtual source and receiver. For energy arriving from distant sources, the auto-correlation can be related directly to the structural reflectivity beneath the receiver. We show how the auto-correlation of seismic signals can provide information on reflections without conversions, and can be exploited to provide indirect imaging of heterogeneous structure. Correlations between signals at different sensors can also be exploited in reflection work to provide virtual sources that provide new ways of imaging complex structure.