The first level of organization are living cells, the smallest units of all living things. The second level of organization are tissues, which are groups of cells working together. The next level of organization is an organ, groups of tissues working together to perform a specific activity in the organism. Then there are organ systems, which are groups of organs that work in unison to perform a specific function for the body. Lastly, an organism, a group of organ systems put together to make a fully functional multicellular organism.