This is the third of six volumes collecting significant papers of the distinguished astrophysicist and Nobel laureate S. Chandrasekhar. His work is notable for its breadth as well as for its brilliance; his practice has been to change his focus from time to time to pursue new areas of research. The result has been a prolific career full of discoveries and insights, some of which are only now being fully appreciated. Chandrasekhar has selected papers that trace the development of his ideas and that present aspects of his work not fully covered in the books he has periodically published to summarize his research in each area. This volume is divided into four sections. The first, on dynamical friction and Brownian motion, includes papers written after Chandrasekhar published his 1942 monograph Principles of Stellar Dynamics. Also in this section is Stochastic Problems in Physics and Astronomy, one of the most cited papers in the physics literature, as well as papers written jointly with John von Neumann that have been given impetus to recent research. As Chandrasekhar notes, the papers in the second section, on statistical problems in astronomy, were influenced by Ambartsumian's analysis of brightness in the Milky Way. A third section on the statistical theory of turbulence addresses issues still unresolved in fluid dynamics, and the last section is devoted to hydromagnetic problems in astrophysics that are not discussed in Chandrasekhar's monographs.