The first cell seen under a microscope was that of a cork, comprised of dead plant cells. In 1665 Robert Hooke was the person to first see these plant cells under a microscope.
Robert Hooke was observing the cell walls from the dead plants but was unable to detect the smaller substructure of the cell, known as organelles. He was the one to remark that these smaller units of life building looked very similar to the cells that monks lived in, hence the name. However, it wasn't until 1674 that Anton van Leeuwenhoek first saw living cells in algae. The cell theory, which says that all life is made up of smaller units, was officially established in 1839 and still forms the basis for modern molecular biology.