Cerebrospinal fluid (CSF) is a clear, colourless ultrafiltrate of plasma with low protein content and few cells. The CSF is mainly produced by the choroid plexus, but also by the ependymal lining cells of the brain's ventricular system. CSF flows through the ventricular system and then into the subarachnoid space and it is subsequently absorbed through the subarachnoid villi into the venous system. CSF has several functions in the nervous system. It protects the brain during blood pressure fluctuations, regulates the chemical environment of the central nervous system and it is a vehicle for intracerebral transport. This two-part article reviews CSF function, physiology, analytical techniques and interpretations in disease states of companion animals. This first part will address the function and composition of CSF in companion animals.