  The HBT method is used to get information about the sizes, shapes and
sometimes also about the time evolution of the homogeneity regions in
hadroproduction processes. Homogeneity region K is the region where the hadrons
with momentum K are produced. The size and shape of he homogeneity region is
described by the Wigner function W(K,X) evaluated in the interaction picture
after all the hadrons had been produced. Additional information about the
interaction region is contained in the emission functio S(K,X). A theorem is
presented and discussed which specifies which of the parameters characterizing
the Wigner functin can and which cannot be obtained using the HBT method. In
order to obtain the complete Wigner fuction additional assumptions are needed.
For instance, it is enough to know the distribution of the centers of the
homogeneity regions <X>_K. In order to find the emission function further
assumptions are required. No systematic analysis is available, but some
instructive examples are discussed.
