Electrical impedance in cochlear implants provides a unique perspective on the interface between engineering and living tissue. This chapter traces how this fundamental electrical property is used as a powerful clinical and research tool, capable of revealing critical insights into cochlear implant function, tissue response, and surgical outcomes. We begin by outlining the biophysical bases of impedance, including the roles of resistive and reactive components, and how they influence signal behavior across time and frequency domains. These principles are then applied to the context of the cochlea, where impedance measurements reflect the dynamic relationships between the electrode array and the surrounding biological environment. We explore how impedance telemetry has become central to monitoring electrode status, detecting device malfunctions, and guiding programming strategies. Through modeling approaches and spectral analyses, we highlight how impedance data can provide a deeper understanding of the electrode-tissue interface. The chapter also reviews current clinical protocols across the main manufacturers, including the use of electrode electrical field potentials for spatial mapping. Finally, we examine emerging techniques that are extending the clinical utility of impedance measurements. Together, their advancements are transforming impedance telemetry from a basic safety check into a rich source of diagnostic and prognostic information in cochlear implantation.