This Research Topic is part of the Ear-Centered Sensing: From Sensing Principles to Research and Clinical Devices series: From Sensing Principles to Research and Clinical Devices, Volume I The human ears are an attractive location for bio-signal acquisition. Heart rate, respiratory rate, eye blink and eye motion signals and skin conductance, as well as the electrical activity from muscles and the brain can be recorded from the ear. Moreover, the ears provide a discreet and natural anchoring point for placing the necessary wearable hardware, thereby reducing the visibility of integrated devices. We define ear-centered sensing as monitoring physiological signals with sensors located in the ear canal, in the pinna, or around the ear. Ear-centered sensing allows data recording over extended periods of time in everyday situations with little disturbance for the users. The combination of physical measurements such as motion, temperature and moisture, and electrophysiological measurements, such as electroencephalography (EEG), electrocardiography (ECG), electromyography (EMG), electrooculography (EOG), and electrodermal activity (EDA), for example, integrated over long time periods, will help to gain a better understanding of psycho-physiological processes. Ear-centered sensing is therefore of interest for scientific, diagnostic and therapeutic purposes and we believe that it will play a significant role in future mobile health applications. As the ear is an unconventional place for monitoring these physiological measures, a common challenge for ear-centered sensing is to gain a better understanding of the signals that are recorded at this location. The questions that need to be answered are: How does the signal (e.g. ECG, or EEG) acquired at the ear relate to the signal as acquired at the classical recording sites? Which signals are ear-centered systems sensitive to, which signals are lost? How can we reliably discriminate in real time signals from artifacts? And finally, how do we interpret data that is acquired over extended periods of time when we have little or no control over the recording environment? For the sensing of physiological signals over extended periods of time dedicated sensor and amplifier technology is needed that is convenient to use, robust and reliable. People wearing these sensors should not be restricted in their activities. Hence, for long-term usage sensor and amplifier technology need to be unobtrusive in every aspect: the materials need to be biocompatible, adjust to the individual's anatomy and be comfortable to wear. They need to be sufficiently robust to allow for continued usage and self-fitting, and they need to be small and inconspicuous. The electronic instrumentation, including bio-signal conditioners and amplifiers, analog-to-digital converters, means for signal processing and wireless transmission need to be sufficiently small and light-weight to be placed at the ear together with the sensors. The power supply has to be secured either by low-power electronics or by smart ways to recharge the battery, or even by harvesting body energy. For the tiny signal changes, as produced for example by brain activity amplifiers need to be sensitive enough to detect them while maintaining robust artifact rejection capabilities.