One of the goals of exploration geophysics is to obtain an image of the subsurface. In petroleum exploration and near-surface geophysics, this is best achieved using reflected waves. For this, a controlled seismic or electromagnetic source is placed at the surface, activated, and the wavefields that it creates are recorded at the surface after being reflected from subsurface structures. However, it is not always possible to use controlled sources at the surface. An alternative is to make use of seismic or electromagnetic interferometry. By crosscorrelating wavefields, which are recorded at two points at the surface and result from sources around the two points, the reflection response can be retrieve between these two points as if there were a controlled source at one of the points and a receiver at the other. This book shows derivations of different seismic and electromagnetic interferometry relations using as a starting point two-way and one-way reciprocity theorems of the time-correlation type. Using numerically modeled data, the retrieval of the reflection response is investigated from the crosscorrelation of recorded wavefields from subsurface transient and noise sources. This is performed for acoustic, elastic and electromagnetic waves. The ability of seismic interferometry to retrieve reflection responses in practical applications is demonstrated with two examples. The first example uses laboratory data from an inhomogeneous granite block, where transmission recordings are crosscorrelated resulting from separate P-wave and S-wave transient sources. The second example is with field data from a desert area, where transmission recordings are crosscorrelated resulting from seismic background-noise sources in the subsurface.