The electrically evoked compound action potential (ECAP) is a synchronous response of the auditory nerve to stimulation with a current pulse from a cochlear implant electrode. The cochlear implant platform offers a unique opportunity to record neural activity using hardware very close to the nerves, as the electrodes of the device used to stimulate and evoke an auditory percept can also be used to record the neural response. ECAP measures are used clinically, especially in cases where it is not possible to assess auditory perception behaviorally. They can provide insight into patient-specific patterns of neural responsiveness and the interface between the electrodes and the auditory nerves more generally. The ability of the ECAP to predict for biophysical or behavioral indicators of auditory perception is, however, limited by its recording techniques and the fact that it only captures the response at the very periphery of the auditory system. Nonetheless, it is the most widely used objective measure of neural activity in CI healthcare, especially as it is simple to measure clinically. This chapter covers methods for recording ECAPs, interpretation of the measurement waveform, relationships to other methods of assessing CI perception, and extended techniques for modeling the electrode-neuron interface.