In this chapter, the concept of physiological noise is formally introduced. Beginning with the assumption that a physiological system can be represented as a dynamical system governed by an unknown analytical map, physiological noise is modeled as dynamical noise acting upon this system—i.e., as a stochastic component intrinsically embedded in its dynamics. The chapter then provides an anatomical overview of the physiological systems examined in this book, with a focus on neuro-autonomic cardiac control and neural systems across multiple scales, including individual neurons, the brain, and the cerebral cortex. For each of these, the signals used for investigation are discussed, along with the primary sources of physiological noise inherent to each system.