The most basic unit of life according to cell theory is the cell. Cell theory states that all living organisms are composed of one or more cells, which are the smallest units of life capable of performing essential biological functions. Cells can exist as unicellular organisms or as part of multicellular entities, where they perform specialized roles. The understanding of cells as the fundamental building blocks of life forms the basis for numerous biological fields, including microbiology, genetics, and physiology. Cells also replicate and transmit genetic material, providing continuity of life.