Virtual black holes and the S-matrix

A brief review on virtual black holes is presented, with special emphasis on phenomenologically relevant issues like their influence on scattering or on the specific heat of (real) black holes. Regarding theoretical topics, the results important for (the avoidance of) information loss axe summaxized. After recalling Hawking's Euclidean notion of virtual black holes and a Minkowskian notion which emerged in studies of 2D models, the importance of virtual black holes for scattering experiments is addressed. Among the key features is that virtual black holes tend to regularize divergences of quantum field theory and that a unitary S-matrix may be constructed. Also, the thermodynamical behavior of real evaporating black holes may be ameliorated by interactions with virtual black holes. Open experimental and theoretical challenges are mentioned briefly.