How does one design something that is complex? Or something that is simple? Why should one try to reduce or increase complexity? What is complexity? There are a large number of different uses of the word, including many in mathematics and physics. Most of these are not useful in attempting to fit the word to the problems of the design of systems and devices for medicine. In this paper the concept has been defined to apply to health care, which has led to some conclusions about the future development of medical systems and devices.