Cerebrospinal fluid (CSF) is a clear and colourless fluid that surrounds the brain and spine. Due to the close proximity of CSF to the brain, pathological brain-processes are likely to be reflected in CSF. CSF can be obtained through lumbar puncture and is frequently performed in the differential diagnosis of neuropsychiatric disorders. Beyond clinical applications, CSF has been studied as part of different research-protocols. In this review, we will focus on CSF-analysis in Alzheimer Disease, major depression and schizophrenia. We will review both clinical applications as well as research applications in all three disorders. We will also assess new technological advances that have made it possible to study large numbers of proteins in CSF and how these advances may change CSF-analysis in the years to come.