This thesis is about the exciting and difficult field of health informatics. First it is a thesis about the professional workers like secretaries, nurses and doctors, and aspects regarding their field of knowledge. In the society of today this includes their use of technological apparatus. Information technological actors (like Electronic Patient Records) are thereby important in the construction and communication of knowledge. Secondly the individual citizens and their relatives are increasingly knowledgeable, and are often able to challenge the professional health workers in their thinking and acting. The patient is today seen as a consumer with rights, and everyone is supposed to use integrity and rationality in their interaction with professional experts. The professional strives in continually updating their professional knowledge, while the patients use their individual rights, and their knowledge of individual aspects. The challenge is then to avoid a break between different interests and knowledges: the professional has to maintain integrity while cooperating with the patients. But as John Law (one of the ANT founders) says: perfectionism is dangerous.