The perfect storm of available data for diagnosis and therapy (Big Data) can only be mastered by assistive cognitive systems. Intelligent algorithms can be used to deliver information and evidence transparently to the doctors and medical staff so that the patient receives the best possible care. Insights have to be generated out of data so that meaningful solutions can be offered. Cognitive systems can understand language (spoken and written), images and they can make use of structured and unstructured data (e.g. medical texts) exhaustively. Cognitive Systems not only understand information but also put data into context and understand the underlying concepts. Cognitive Systems learn continuously from the growing medical knowledge and the interaction with experts. Therefore, they get better from day to day. Two key points are important: The IBM Watson technology has been developed to allow a dialogue with the user and to represent data, insights and reasoning in a transparent manner. That way, the user can access and comprehend the information fast and understand how the system came to a certain conclusion or hypothesis. Cognitive Computing will become an essential part of medicine because the insights that are relevant for diagnosis and therapy have to be gained from the overload of medical information which is available today. And the flood of knowledge and data will increase in the years to come. Cognitive Computing will NOT make the physician redundant. Quite opposite, cognitive systems will assist and serve physicians and medical staff and be a catalyst in the digital revolution of medicine.