Boltzmann’s struggle with a derivation of the Second Law of Thermodynamics is sketched. So is his first derivation of the connection between entropy and probability in 1877. Planck’s derivation and quantum mechanical modifications of Boltzmann’s connection between entropy probability are given next. Then Einstein’s objections to a purely probabilistic rather than a dynamical interpretation of entropy are discussed. Finally, the dynamical basis of the Sinai–Ruelle–Bowen distribution for very chaotic systems is sketched and appears to be an example of Einstein’s dynamical interpretation of entropy.