Ultrafast molecular imaging represents an emerging frontier.In particular, recent developments in the ultrafast electron diffraction (UED) have demonstrated the ability to image the rearrangements of chemical bonds in complex systems with resolutions of ~0.01A and ~1 ps, respectively. These new limits provide the means for the determination of transient structures of molecules, surfaces and nanostructures, including reactive intermediates and nonequilibrium structures of complex energy landscapes. By freezing structures on the ultrafast timescale, we are able to develop concepts that correlate structure with dynamics. Examples include structure-driven radiationless process, and nonequilibrium structures exhibiting negative temperature, bifurcation, or selective energy localization in bonds. Recent development of ultrafast electron crystallography for studying condensed phase reactions and phase transitions in the nanometer scale reveal the transient phenomena at interfaces and in nanophases. Atomic scale processes of both coherent and incoherent energy conversions through carrier-phonon, phonon-phonon and configurational interactions in systems of finite size were imaged. In this talk, I will highlight the advances made and the future perspectives for the UED.