Three groups of cerebrospinal fluid (CSF) cells can be formed: 1. cells of the normal CSF, such as monocytes, small lymphocytes and occasionally cells of the ventricle system, 2. cells found in dogs and cats with neurologic disorders, such as reactive monocytes and lymphocytes, macrophages, neutrophils and eosinophils in addition to cells of the first group, 3. neoplastic cells. The different cells are introduced and their origin, function and occurrence are discussed. Mitotic figures, degenerated cells and artefacts are also mentioned.