The invention of the electron microscope is generally attributed to 1931, when Ernst Ruska and Max Knoll built the first working prototype at the University of Berlin.

However, it's important to note that:

* Earlier work by other scientists contributed to its development. For example, Hans Busch laid the foundation for the technology in the 1920s by demonstrating the focusing of electron beams with magnetic fields.

* The electron microscope evolved significantly over time. The first models were very rudimentary, and it took several years for the technology to be refined and become widely adopted.

Therefore, while 1931 is widely regarded as the year of its invention, it's a complex story with contributions from multiple scientists over a period of time.