Multicellular organisms are those organisms containing more than one cell, and having differentiated cells that perform specialized functions. Although some single-celled organisms have differentiated cells (e.g. myxobacteria), the differentiation is less dramatic than that typically found in multicellular organisms. Multicellular organisms must solve the problem of regenerating the whole from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. The overall spatial organization of differentiated cells is a topic of study in anatomy.

Multicellular organisms can suffer from cancer when cells fail to regulate their growth within the normal program of development.