  I review recent progress in the understanding of the connection between the
space-time structure of the particle emitting source and the form of the
two-particle correlation function in momentum space. Based on a new scheme for
calculating the HBT radius parameters from the emission function, strategies
are suggested to separate for rapidly expanding sources the information on the
spatial and temporal structure of the source. To this end a new fitting
function for the two-particle correlation function is proposed. Its usefulness
is demonstrated for a typical expanding model source, and it is shown how the
dependence of the resulting fit parameters on the momentum of the particle pair
can be used to measure the longitudinal and transverse expansion of the source.
