  Bose-Einstein correlations in relativistic heavy ion collisions are examined
in a general model containing the essential features of hydrodynamical, cascade
as well as other models commonly employed for describing the particle
freeze-out. In particular the effects of longitudinal and transverse expansion,
emission from surfaces moving in time, the thickness of the emitting layer
varying from surface to volume emission and other effects are studied. Model
dependences of freeze-out sizes and times are discussed and compared to recent
$Pb+Pb$ data at 160A$\cdot$GeV.
