Antoine van Leeuwenhoek invented the microscope. His microscope was very simple, and he made all the microscope parts himself. Although his microscope didn’t offer the greatest magnification, he was the first person to see many, many micro-organisms, and describe them.
Be sure to check out my free book to get you started with microscopes on the main page of this website!
In the meantime check out the incredible video telling the Antoine van Leeuwenhoek story. This video is by the New York Times.