As with stop-motion photography and flash stroboscopy, in 4D imaging the molecular motion has to be resolved into frames using a sequence of flashes, in our case the probing electron pulses. However, 4D electron imaging demands the marriage of ultrafast probing techniques with those of conventional microscopy and diffraction, as well as the development of concepts describing the simultaneous temporal and spatial resolutions of atomic scale. Since the first reports from this laboratory, the technical and theoretical machinery had to be further developed, and currently at Caltech there are five table-top instruments for studies of gases, condensed matter and biological systems. Examples of the experimental setups for UED, UEC and UEM are displayed below. The conceptual framework of the approach is as follows. Upon the initiation of the structural change by either heating of the sample, or through electronic excitation induced by ultrashort laser pulses, a series of electron pulses is used to probe the specimen with a well-defined time delay. An electron diffraction pattern or a micrograph is then obtained. A series of images recorded at a number of delay times provides a direct insight into the temporal evolution of the structure. Isolated reactions are studied by using collisionless molecular beams (UED), while crystals and surfaces are examined either in the reflection or transmission mode (UEC). For microscopy, the electron beam typically penetrates a nm-scale sample (UEM). Because electrons strongly interact with air, the 4D imaging measurements are usually performed in vacuum.