* 1900 – Max Planck suggests that light may be emitted in discrete frequencies, giving his law of black-body radiation
* 1905 – Albert Einstein, in the first of his miracle year papers, argues that the reality of quanta would explain the photoelectric effect
* 1905 – Einstein mathematically analyzes Brownian motion as a result of random molecular motion in his paper On the movement of small particles suspended in a stationary liquid demanded by the molecular-kinetic theory of heat
* 1906 – Nernst presents a formulation of the third law of thermodynamics
* 1907 – Einstein uses quantum theory to estimate the heat capacity of an Einstein solid
* 1909 – Constantin Carathéodory develops an axiomatic system of thermodynamics 
* 1910 – Einstein and Marian Smoluchowski find the Einstein–Smoluchowski formula for the attenuation coefficient due to density fluctuations in a gas
* 1911 – Paul Ehrenfest and Tatjana Ehrenfest–Afanassjewa publish their classical review on the statistical mechanics of Boltzmann, Begriffliche Grundlagen der statistischen Auffassung in der Mechanik
* 1912 – Peter Debye gives an improved heat capacity estimate by allowing low-frequency phonons
* 1916 – Sydney Chapman and David Enskog systematically develop the kinetic theory of gases
* 1916 – Einstein considers the thermodynamics of atomic spectral lines and predicts stimulated emission
* 1919 – James Jeans discovers that the dynamical constants of motion determine the distribution function for a system of particles
* 1920 – Meghnad Saha states his ionization equation
* 1923 – Debye and Erich Hückel publish a statistical treatment of the dissociation of electrolytes
* 1924 – Satyendra Nath Bose introduces Bose–Einstein statistics, in a paper translated by Einstein
* 1926 – Enrico Fermi and Paul Dirac introduce Fermi–Dirac statistics
* 1927 – John von Neumann introduces the density matrix representation, establishing quantum statistical mechanics
* 1928 – John B. Johnson discovers Johnson noise in a resistor 
* 1928 – Harry Nyquist derives the fluctuation-dissipation theorem, a relationship to explain Johnson noise in a resistor 
* 1931 – Lars Onsager publishes his groundbreaking paper deriving the Onsager reciprocal relations
* 1935 – Ralph H. Fowler invents the title the zeroth law of thermodynamics to summarise postulates made by earlier physicists that thermal equilibrium between systems is a transitive relation
* 1938 – Anatoly Vlasov proposes the Vlasov equation for a correct dynamical description of ensembles of particles with collective long range interaction
* 1939 – Nikolay Krylov and Nikolay Bogolyubov give the first consistent microscopic derivation of the Fokker–Planck equation in the single scheme of classical and quantum mechanics 
* 1942 – Joseph L. Doob states his theorem on Gauss–Markov processes
* 1944 – Lars Onsager gives an analytic solution to the 2-dimensional Ising model, including its phase transition