This lecture provides a historical overview of Transmission Electron Microscopy (TEM), starting with the first prototype built in 1931 by Knoll & Ruska. It covers the development of TEM, surpassing the light microscope in resolution in 1933, and the first commercial instrument in 1939. The lecture also explains the Abbe diffraction limit for light and electron microscopes, detailing the equivalent parameters for TEM. Furthermore, it delves into the components of a typical TEM, such as the electron gun, emitter, electron lenses, apertures, and specimen holders. The importance of apertures in defining beam properties and contrast formation is highlighted.