Prion diseases of humans and other animals have clinical, economic and political significance. Pre-mortem diagnosis is therefore of great importance. Clinical diagnostic criteria and the current status of the available diagnostic tests are reviewed and possible future developments discussed. Presently, most diagnostic tests are indirect, relying on findings that may not be confined to prion diseases rather than on their particular intrinsic nature. They are therefore not absolutely specific nor are they 100% sensitive. The electroencephalogram (EEG), cerebral imaging and cerebrospinal fluid (CSF) analysis are the main techniques employed. However, there is hope that useful blood tests could be developed which would be simpler and less invasive. Also, there is hope that more specific tests will become available.