Abstract: Proteins carry out an enormous array of functions, from chemical catalysis over molecular sensing to the efficient interconversion of chemical, mechanical, electrical, and light energy. To understand how proteins function, we study their dynamics with molecular simulations that cover a wide range of temporal and spatial scales. At one extreme, we follow fast, photoexcitation-driven protein motions using a hybrid quantum-mechanics/molecular-mechanics (QM/MM) description. The resulting simulation trajectories are compared directly to femtosecond time-resolved protein crystallography and solution scattering experiments at an X-ray free electron laser (XFEL), and to femtosecond optical spectroscopy. At intermediate scales, we study the functional dynamics in molecular rotary motors and pumps using classical molecular dynamics simulations. At the slow extreme, we combine atomistic and coarse-grained simulations to find out how eukaryotic cells probe the physical characteristics of their lipid membranes, and in response activate regulatory processes. For these systems, and for biomolecular machines in general, molecular dynamics simulations provide us with critical insight into the molecular principles underlying their efficient operation. Biosketch: Gerhard Hummer received his PhD in physics in 1992 for work at the University of Vienna, Austria, and the Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany. He joined the Los Alamos National Laboratory, first as a postdoctoral fellow (1993-1996) and then as a group leader (1996-1999). In 1999, he moved to the National Institutes of Health, where he became Chief of the Theoretical Biophysics Section, and Deputy Chief of the Laboratory of Chemical Physics, NIDDK. In 2013 he joined the Max Planck Institute of Biophysics in Frankfurt, Germany, as director of the newly formed Department of Theoretical Biophysics. Since 2016, he is also Professor of Biophysics at the Goethe University in Frankfurt. Dr Hummer uses molecular simulations, modeling, and theory to study the structure and dynamics of biological systems at the molecular level, in an effort to elucidate their function. His current research focuses on molecular principles in bioenergetics, membrane transport, and membrane remodeling. Gerhard Hummer is a Fellow of the American Physical Society (2005) and received the Raymond and Beverly Sackler International Prize in Biophysics (2010).