In biology, cell theory is a theory that describes the properties of cells, the unit of structure in every living thing. The development of the theory, during the 17th century, was made possible by advances in microscopy; the study of cells is called cell biology.

A cell is one of the smallest units of living matter. All living things including plants and animals are made up of cells. Cells are made of atoms, which are the smallest units of matter. There are many different kinds of cells. The two kinds you are most likely to be familiar with are animal and plant cells.

Plant and animal cells have several differences. For example, animal cells do not have a cell wall or chloroplasts but plant cells do. Animal cells are round in shape while plant cells have fixed, rectangular shapes.

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using this instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.

In a wet mount, the cell is placed in a drop of water held between the slide and the cover slip by surface tension.

In a dry mount, the object is placed on the slide. A cover slip may be placed on top to protect the specimen.