Inspite of considerable progress in prevention, diagnosis, and treatment, pulmonary embolism has remained a threat to the patient and a challenge for the physician both in conservative, as well as in operative disciplines. Pulmonary embolism is according to pathology observations still the most frequently overlooked clinical diagnosis. In 1-5 per 100 autopsies, clinically unexpected pulmonary emboli are found. In addition, the sequelae of recurrent pulmonary emboli, the syndrome of pulmonary hypertension with or without right heart failure, continues to present a therapeutic dilemma - and no progress is in sight. In intensive care medicine pulmonary embolism, either acute, massive, and/or recur rent, continues to be both a therapeutic as well as a preventive challenge mobilizing pharmacotherapeutic, catheter-interventional, and operative resources. Diagnostic, therapeutic, and preventive strategies are currently in use. Their basis, however, seems surprisingly thin, as far as our knowledge on the natural course of this chameleon-like illness with and without fibrinolytic, anticoagulative, catheter or opera tive treatment is concerned. A large European multicenter register has been initiated by Professors Kasper and Geibel with the help of Boehringer Ingelheim Pharmaceutics, in order to better describe the natural course of pulmonary embolism under current treat ment modalities. Furthermore, recently the clinical significance of the valve patent foramen ovale as a source of paradoxical emboli is beginning to be better understood. Many concepts therefore require revision.