Here's a timeline:

* 1590s: Zacharias Janssen and Hans Lippershey are credited with the invention of the compound microscope.

* 1665: Robert Hooke observed and described the structure of cork using a microscope, coining the term "cell."

* 1674-1676: Antonie van Leeuwenhoek, using his own single-lens microscopes, observed and described bacteria, yeast, and red blood cells.

So, while microscopes were invented in the late 16th century, it was in the 17th century that they were first used to study and understand the basic building blocks of living organisms: cells.