Proposed by Mathias Jacob Schleiden, 1838 (Jerman Botanist), Theodore Schwann, 1839 (Jerman Zoologist)
It consists of 5 fundamental facts -
(i) Body of living organisms consists of different types of cells and their products i.e. Cell is the structural unit of life.
(ii) Single cell perform all the vital activities i.e. cells is the functional unit of life. All the activities of living organisms occur due to the metabolic activities of the cell.
(iii) Cell is the hereditary unit of life.
(iv) Cells are living or cell has life.
(Each cell is made of a small mass of protoplasm containing a nucleus in its inside and a plasma membrane with or without a cell wall on its outside)
(v) New cells arise from pre-existing cell. This statement was supported by "Cell Lineage Theory of Rudolf Virchow" which stated "Omnis cellula e cellula" i.e. new cells are formed inside the preexisting cells.