Anton van Leeuwenhoek was the first scientist to closely observe cells under a microscope; he paved the way for a modern understanding of biology overall. He actually gave cells their name after the resemblance he believed they had to a monk's quarters. Anton van Leeuwenhoek is considered to be the father of microbiology.
Anton van Leeuwenhoek was a very prolific scientist and had a very long life, dying at the age of 91. One of his most ground-breaking discoveries was also one of his first. His observations, in 1674, of scummy pond water led to the first visual descriptions and illustrations of such common organisms as the algae spirogyra. This contributed towards the foundation of several sub-fields of biology.
A few other important scientific milestones of van Leeuwenhoek's include the discovery that baker's yeast consists of tiny plant-like organisms; the discovery of motile bacteria in tartar on human teeth; and, what he considered his greatest discovery of all -- the direct observation of sperm cells in the semen of humans, dogs, swine, mollusks, amphibians, fish and birds. This discovery is particularly amazing considering the state of technology at the time, as sperm are some of the smallest cells in the human body.