The most obvious evidence of cell theory is to look at cells themselves. Some cells are large enough to be seen with the bare eye; amoebas, for example, can be seen with a backlight. Using a magnifying glass or a microscope, even more kinds of cells can be seen, such as plant cells and skin cells. Some, like red blood cells and bacteria, are even smaller.

More evidence can be found in the behavior of a cell on it's own. An individual cell can be alive without the host body in the right conditions, and with microscopes these can be seen.

Further, cell replication and organelles can be seen through microscopes. Onion skin cells are commonly used to show cell replication, and some algae have easy to see organelles.

Cell Theory states that

• all living beings are made up of cells
• cells come from pre-existing cells
• a cell is a structural and functional unit of life and is also responsible for heredity 
• a cell is the smallest unit of life.

As with any other theory, evidence of cell theory came from observations and experimentation. Robert Hooke, in 1663, observed the first cell on a cork and coined the term 'cell'. Anton van Leeuwenhoek (1632-1723) made numerous observations of living cells, including blood cells and sperm cells. Lorentz Oken, in 1805, stated the first postulate of cell theory (all living being...of cells). Robert Brown, around 1833, discovered the nucleus. Schwann (with help from Schleiden), in 1838, stated the two key postulates of cell theory (actually he did 3, but the third was proven wrong). Rudolph Virchow stated that all cells came from pre-existing cells.

Cell theory is based on the works of these scientists and has been refined over time.