The study of cells is greatly facilitated by growing them in culture, so that they can be examined by microscopy and subjected to specific treatments under controlled conditions. It is generally quite easy to grow unicellular bacteria, fungi, or protists, for example, by placing them in a rich medium that supports their growth. However, animal cells come from multicellular organisms, which makes it more difficult to culture single cells or small groups of cells. In this section, we discuss how animal cells are grown in culture and how different cell types can be purified for study.