How does a sperm cell find the ovum? How do white blood cells direct themselves to the site of injury or inflammation? How do unicellular microorganisms find their food and avoid hostile environments? The common answer to all these questions is — by chemotaxis. Similarly to a butterfly that is attracted to a flower by odor, or a male insect that is attracted to the female by pheromones, many organisms (primarily, but not solely, unicellular) and cells of multicellular organisms are attracted to their targets or repelled from certain chemicals by chemotaxis. The book deals in depth with a number of different chemotaxis systems and looks for similarities and diversities between them.