This book presents recent advances in theory and experimental methods in electron diffraction. A set of spectral and diffraction methods using different complementary physical principles that allow for the photo-excitation dynamics and diagnostics of nuclei and electrons in the temporal sequences of ultrashort duration are detailed. New research opportunities that provide the required integration of the triad: "Structure - Dynamics - Function" in chemistry, biology, materials science are explored. This reference will be of interest to a wide range of readers interested in the problems of the structure of matter, including the study of the structure and dynamics of nanomaterials.