A system is a unified singularity which is composed of many interrelated elements that are substantively or conventionally selected. In this chapter we shall discuss biosystems, focusing specifically on the living bodies of cells and multicellular organisms. As previously stated, it should be clear that not all biosystems are living. For example, the population of organisms is a biosystem, but is not itself considered living. The nucleus of a cell is also a biosystem, but once again is not a living body.