The author recalls that since Quinke, in 1891, directed the attention of the Wiesbaden Congress to the use of lumbar puncture, numerous publications have appeared on the subject. He observes that the high expectations of the therapeutic value of this operation have not been realised, any improvement following being seldom lasting. It has been used with most effect in acute, serous, and sero-purulent meningitis, and less so in simple hydrocephalus, brain tumour, and tubercular meningitis. Lumbar puncture has, however, been an important addition to our means of diagnosis. Through it we are able to ascertain whether there is abnormal increase in the spinal fluid. This gives an important indication in cases of brain tumours and of serous meningitis with obscure symptoms. In doubtful cases of tubercular meningitis the detection of tubercle bacilli in the spinal fluid determines the diagnosis of the disease.