In situ electron microscopy enables the site-specific correlation between atomic structure and properties. Developments of capabilities to manipulate and stimulate by, for example, electric fields, light, mechanical strain and temperature in the electron microscope allow quantitative studies of the active atomic structure using different imaging, spectroscopy and diffraction modes. The spatial resolution of the imaging mode is better than 1 Å and the precision in determining the position of atoms can be better than 1 pm. This enables the study of role of not only material phases but also individual atoms, molecules, defects and interfaces in determining the properties. Electron tomography is used to determine the 3-dimensional structure of the materials. The obtained knowledge is used to tune the properties of advanced materials and devices. Catalytic activity of metal nanoparticles and electrical properties of semiconducting nanowires are examples where the strain induced effects have a strong influence on the properties and performances. Electric fields can change the surface structure of materials and the thermal handling capabilities can be changed by the presence of a one atomic layer thin surface film. New aspects of material properties and mechanisms, not obvious from measurements on the macro scale, can be revealed using the high resolution and in situ electron microscopy. The knowledge is crucial for not only the understanding of the mechanisms that are involved but also for the design of materials and devices with tailored properties.