The invention of and subsequent refinements of the microscope led to the eventual ability to see cells. Robert Hooke was the first person to see evidence of cells in living things. In 1665, using a primitive microscope, he observed cell walls in a slice of cork. He named these spaces "cells", from the Latin word cellulae which means small spaces or small rooms.
In 1670 Antonie van Leeuwenhoek became the first person to observe living cells, describing Protozoa in pond water and bacteria from saliva. He built microscopes from lenses that he ground himself and polished to improve magnification. He was able to reach magnifications of up to 275X. Van Leeuwenhoek continued to study the microscopic world and observed many other types of cells, including human red blood cells and sperm cells.
The microscope later led to the discoveries in 1835-1836 that all plants and all animals are made of cells, by Matthias Schleiden and Theodor Schwann respectively. These discoveries were the basis for the Cell Theory developed by Schleiden and Schwann.