  By measuring hadronic single-particle spectra and two-particle correlations
in 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 2-3 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.
