Biological systems are made up of interacting chemical and physical processes. Living systems are composed of many subsystems and components, each having its own unique characteristics and behavior while contributing to the overall form and function of an entire system. These systems are highly complex; many components interact simultaneously, and their interactions are highly nonlinear or chaotic in nature. These interactions and nonlinearities must be taken into account when attempts are made to understand or predict system behavior. Our understanding of these interactions is incomplete and often guided only by empirical evidence of overall system behavior instead of empirical data on processes that lead to overall system behavior. Because of these complexities, the classical mathematical methods used to study nonliving physical or chemical systems have been inadequate for living systems.