Chemical reactions and phase transformations in nanometer-size objects may happen at very short timescales, so conventional characterization tools are not applicable. A new technique combining high spatial and temporal resolution is ultrafast (or dynamic) electron microscopy, where short electron pulses are used instead of continuous electron beams. In a pump-probe approach, fast transformations of the objects are induced by laser pulses and monitored by short photoelectron pulses in a transmission electron microscope. This approach allows us to study fast reaction kinetics in individual nanoparticles and detect short-living transient states. First, the article briefly introduces transmission electron microscopy techniques and techniques for creating short electron pulses, finally leading to the principle of ultrafast TEM. In the second part, applications of ultrafast TEM in different fields of nanoscience are discussed with a focus on nanochemistry, where the analysis of nanoparticles with nanosecond resolution is realized.