In 1955 Wolf noticed that the mutual coherence function Gamma obeys two wav
e equations [Proc. R. Soc. London 230, 246 (1955)]. The physical optics of
this finding is thoroughly presented in Mandel and Wolfs recent monograph [
Optical Coherence and Quantum Optics (Cambridge U. Press, Cambridge, UK 199
5)]. We discuss those coherence waves in the spirit of optical signal proce
ssing. The term coherence waves is justified if one accepts the following p
aradigm: A wave is whatever obeys the wave equation. (C) 1999 Optical Socie
ty of America [S0740-3232(99)00902-3].