  A comprehensive investigation is carried out on the class of energy based
ensembles. The eight ensembles are divided into two main classes. In the
isothermal class of ensembles the individual members are at the same
temperature. A unified framework is evolved to describe the four isothermal
ensembles. Such a description is provided both in the second and the third
constraint formalisms. The isothermal- isobaric, grandcanonical and the
generalized ensembles are illustrated through a study of the classical
nonrelativistic and the extreme relativistic ideal gas models. In the adiabatic
class of ensembles the individual members of the ensemble have the same value
of the heat function and a unified formulation to described all the four
ensembles is given. The nonrelativistic and the extreme relativistic ideal
gases are studied in the isoenthalpic-isobaric ensemble, the adiabatic ensemble
with number fluctuations, and, the adiabatic ensemble with number and particle
fluctuations.
