  I review recent applications of two-particle intensity interferometry in high
energy physics, concentrating on relativistic heavy ion collisions. By
measuring hadronic single-particle spectra and two-particle correlations in
hadron-hadron or heavy-ion collisions, the size and dynamical state of the
collision fireball at freeze-out can be reconstructed. I discuss the relevant
theoretical methods and their limitations. By applying the formalism to recent
pion correlation data from Pb+Pb collisions at CERN we demonstrate that the
collision zone has undergone strong transverse growth before freeze-out (by a
factor 2 in each direction), and that it expands both longitudinally and
transversally. From the thermal and flow energy density at freeze-out the
energy density at the onset of transverse expansion can be estimated from
conservation laws. It comfortably exceeds the critical value for the transition
to color deconfined matter.
