The cell theory is a fundamental principle in biology that describes the basic unit of life and the organization of living organisms. It consists of three main principles: All living organisms are composed of cells: This means that whether an organism is a single-celled organism, like bacteria, or a complex multicellular organism, such as plants, animals, and humans, they are all made up of one or more cells. The cell is the basic unit of structure and function in organisms: The cell is the smallest unit that can perform all the necessary functions of life. Each cell is a complete entity capable of carrying out activities necessary for survival, such as obtaining nutrients, interacting with the environment, and reproducing. All cells come from pre-existing cells New cells are formed by the division of pre-existing cells. This principle is often summarized by the phrase "omnis cellula e cellula, " which means "every cell originates from another existing cell." Cells divide through processes like mitosis (for most cells) or meiosis (for reproductive cells) to create new cells. The cell theory was developed over several centuries by multiple scientists, with contributions from Robert Hooke, Theodor Schwann, Matthias Schleiden, and Rudolf Virchow among others. Their work, observations, and experiments laid the foundation for our understanding of cells as the building blocks of life and the basis for biological organization. Today, the cell theory is a fundamental concept in biology and forms the basis for much of our understanding of living organisms and their functions.