Neurophysiology is a broad discipline involved in the recording and analysis of biological signals of multiple modalities relevant to the nervous system. Neurophysiological studies have contributed substantially to the understanding of the neurobiological underpinnings of psychiatric disease. In clinical psychiatry, EEG and other studies are secondary to the clinical assessment for a reliable diagnosis and prognostication of psychiatric disease, and should not be interpreted in isolation. However, in some clinical situations, such as non-epileptic seizures, they can be confirmatory of the diagnosis. In this chapter, we start with an overview of basic principles in neurophysiology, from cellular and molecular to systems neuroscience. We then describe the technical aspects, rationale, indications and limitations of the most commonly used neurophysiological tests in clinically psychiatry. We outline the main neurophysiological abnormalities present in primary psychiatric disorders and in their differential diagnoses (delirium, epilepsy, dementia, focal cerebral lesions and sleep disorders), as well as the neurophysiological effect of psychotropic medications. We also describe recent advances in neuromodulation techniques, linking diagnosis to therapy.