  A number of recent interference experiments involving multiple photons are
reviewed. These experiments include generalized photon bunching effects,
generalized Hong-Ou-Mandel interference effects and multi-photon interferometry
for demonstrations of multi-photon de Broglie wavelength. The multi-photon
states used in these experiments are from two pairs of photons in parametric
down-conversion. We find that the size of the interference effect in these
experiments, characterized by the visibility of interference pattern, is
governed by the degree of distinguishability among different pairs of photons.
Based on this discovery, we generalize the concept of multi-photon temporal
distinguishability and relate it to a number of multi-photon interference
effects. Finally, we make an attempt to interpret the coherence theory by the
multi-photon interference via the concept of temporal distinguishability of
photons.
