Reaction-diffusion dynamics endows cells with the capacity for accurate positioning, control of length, and timing of processes. Protein systems in cells employ different types of spatio-temporal patterns to ensure precise cell division and guide intracellular processes. The length of biopolymers is regulated by the interplay between polymerization kinetics and patterns of molecular motors. Under which conditions protein patterns emerge, and how these patterns are regulated by biochemical and geometrical factors are major aspects of current research. We will discuss general design principles of such biological pattern forming systems.