An essentially statistical theory of thermodynamics is developed on the basis of additivity and conservation laws of fundamental entities of thermodynamics. The theory developed by Dutta (1953-59) stresses first the importance of the statistical theory of estimation in the statistical foundation of thermodynamics. The method of finding the distribution law is based on the principle similar to that of Bayes' rule in probability theory and the method of maximum-likelihood estimation of mathematical statistics. The model used is macroscopic in nature and the laws of microscopic distributions (M-B, B-E and F-D statistics) have been obtained by additional axiom regarding the constituents of the system. The object of the paper is to present an outline of the theory with some of its extensions to quantum macro-system and non-equilibrium thermodynamics. © 1978.