Currently, borreliosis is the most common tick-borne infectious disease in Europe and the USA. In Europe, three species of Borrelia are known to be pathogenic for humans, each of which apparently give rise to different clinical symptoms. Diagnosis is based in particular on the clinical presentation and history of exposure, and, as a rule, is confirmed by serological methods. It must, however, be remembered that the humoral immune response may appear relatively late, so at an early stage, patients are still serologically negative. Depending on the stage involved, oral or peroral antibiotic treatment is applied--however, eradication of the pathogen is unsuccessful in some cases. In future, adjuvant immunotherapy might close the therapeutic gap.