How to see a ghost

This paper discusses various aspects of a solvable quantum model introduced by Friedrichs and Lee in field theory. Its mathematical properties are pre sented, and its role in the early discussion of renormalizability is review ed. It is finally stressed that the model has been of great utility in quan tum optics, and the renown Jaynes-Cummings model is identified as its speci al case. As an illustration it is demonstrated how the Scully-Lamb quantum theory of the laser can be derived from the mathematics of this model. (C) 2000 Elsevier Science B.V. All rights reserved.