All life is made up of cells. Unicellular organisms, as the name suggests, have just one cell, although that cell can be enormously complex. Multicellular organisms contain not just many cells, but critically they contain cells that are different from each other. Originating from a single egg, they develop into different cell types that have different functions. It is this specialization that allows multicellular organisms to develop complex structures such as leaves, eyes and brains. Hence the definition of a multicellular organism put forward by G. Bell and A. Mooers in 1997, that multicellular organisms are clones of cells that express different phenotypes despite having the same genotype. The ‘phenotype’ is the physical characteristics of a cell or an organism that the genes program. For example, in humans, liver and brain and muscle cells all have the same set of genes and all derive from one fertilised egg cell, but they are clearly extremely different. This definition captures the most distinctive property of multicellular organisms. Differentiation and eventually specialization within a group of cells with the same genome leads to the increased complexity that we attribute to multicellular organisms. While all the cells of an individual organism have the same genomes, different sets of those genes are active in different cells within the organism. The only clarification which we might want to add to the pointedly short definition by Bell and Mooers is that this achieved differentiation has to be of a cooperative rather than a competitive nature.