Biological creatures have abilities to generate various kinds of order in a self-organizing manner, and therefore they can be regarded as autonomous distributed systems. When we see biological organisms as autonomous distributed systems, cells are the components of these organisms, because each cell behaves autonomously as an individual entity, taking in substances and energy from the external world, carrying out its own reactions, responding to environmental changes, and even reproducing itself. Moreover, in the case of multicellular organisms, each cell differentiates from others and specializes to have particular functions in order that an assembly of cells can generate complex structures. Such an assembly can act as an autonomous individual that realizes intricate functions which cannot be carried out by individual cells. If we regard individual animals as components, herds of animals are also a kind of self-organizing system. These herds exhibit a variety of complex and clever behavior, in which distinct roles may be allotted to different members of a herd, or in which individuals of different species are in symbiotic relations. The notions of autonomous distributed systems and design by self-organization introduced in the previous chapter were born from abstraction of such biological systems. Mechanisms in biological systems are the result of four billion years of evolution, and in them we can find many clues for artificial system design. In this chapter, we introduce typical and important examples of mechanisms in biological entities.