The central nervous system (CNS) is susceptible to bacterial, viral, and fungal infections, and prion diseases. Examination of the cerebrospinal fluid (CSF) is crucial in diagnosing these infections. Cerebrospinal tests may directly identify an organism and its nucleic acid and surface constituents by culture, polymerase chain reaction (PCR), or antigen detection. Alternatively, antibody to an organism may be identified in CSF by enzyme-linked immunosorbent assay (ELISA), Western blot, or complement fixation assay. This article discusses how these CSF tests are performed and addresses the sensitivity and specificity of such tests for the diagnosis of selected CNS infections.