  The aim of this work is to study the existence of mean values of observables
for infinite-particle systems. Using solutions of the initial value problems to
the BBGKY hierarchy and to its dual, we prove the local, in time, existence of
the mean value functionals in the cases where either observables or states vary
in time. We also discuss problems on the existence of such functionals for
several different classes of observables and for an arbitrary time interval.
