During the last 50 years it has become clear that malignant tumours can induce symptoms unrelated to the mechanical effects of the primary tumour itself or its metastasis. Today, the name Paraneoplastic syndrome is given to those symptom complexes that may affect the blood cells, electrolytes, coagulation system, muscle, skin, nerve and the endocrine system. Endocrine symptoms were first recognised, and different hormones were isolated from the tumour tissue. However, tumour derived hormones are only one of the mechanisms. Thus, especially some neurological syndromes have been shown to be associated with antibodies against neuronal antigens. Some of the antibodies are identified and can be used in clinical praxis. It should also be mentioned that still unknown mechanisms remain to be elucidated. In the present work I have presented an overview over the most common paraneoplastic syndromes. With exception for the neurological paraneoplastic diseases, the symptoms usually disappear if the tumour is removed. Sometimes the paraneoplastic symptoms appear before the tumour itself is large enough to give clinical symptoms. The symptoms must be kept in mind for practical reasons; they may be the first sign of a hidden cancer. Therefore knowledge of these different symptoms may be of importance for a general practitioner for the purpose of detecting a malignant tumour as early as possible. Although I am a medical student with minimal clinical experience, I want to quote the great poet Goethe; “Man sieht nur das, was man weiss”.