  The volume fluctuations in statistical mechanics are discussed. First, the
volume fluctuations in ensembles with a fixed external pressure, the so called
pressure ensembles, are considered. Second, a generalization of the pressure
ensembles is suggested. Namely, the statistical ensembles with the volume
fluctuating according to externally given distributions are considered. Several
examples and possible applications in statistical models of hadron production
are discussed.
