The EU Concerted Action on Risk Assessment in Lyme Borreliosis (EUCALB) has consulted other clinicians and scientists in Europe to produce case definitions of the principal manifestations of European Lyme borreliosis. These case definitions will not only be helpful in supporting its own research interests, but are also intended to assist other clinicians in appropriate management and to support further studies aimed at determining the full clinical spectrum of the disease. The case definitions were achieved after a series of meetings organised by EUCALB with other expert clinicians and scientists from twelve European countries. The definitions and the diagnostic criteria presented thus represent the consensus reached at these meetings. The proposed case definitions consider skin, nervous system, cardiac and musculoskeletal presentations and the role of laboratory investigation in supporting diagnosis.