Biopotentials measurements are essential for biological research and biomedical monitoring of excitable tissues. This chapter provides an overview of biopotential monitoring, from the biological basis to the circuit and system techniques. Signal acquisition, processing, and transmission are fundamental capabilities of biomedical research and medical devices development. In these topics, integrated, low-power consumption systems for portable, wearable, or implantable monitoring of neural signals are taken as study case, presenting both current research trends and established solutions. Starting from the biological sources of the biopotentials, the main observation levels are presented. The design of the integrated front-end amplifier, which must cope with the tougher trade-offs, is discussed, and the system level requirements and alternatives for data acquisition, processing, and wireless transmission are summarized.