All living organisms are made up of cells. This is a fundamental principle of biology known as the cell theory.

Here's a breakdown:

* Unicellular organisms: These organisms are made up of only one cell. Examples include bacteria, archaea, and some types of algae and fungi.

* Multicellular organisms: These organisms are made up of many cells that work together to form tissues, organs, and organ systems. Examples include plants, animals, and most fungi.

It's important to note that viruses are not considered living organisms because they are not made up of cells. They require a host cell to replicate.