This book "Pattern Theory" summarizes the work of an important contemporary thinker, the architect and systems theorist Christopher Alexander, an emeritus professor at the University of Berkeley, California. In 1979 one of his books, "A Pattern Language," an 1100-page-essay on architecture and human life, became a non-fiction best seller that attracted half a million readers and continues to inspire people from many disciplines. The still broader four-volume essay "The Nature of Order" was published 2002-2004 as magnum opus. The life work of Alexander is outstanding. He develops a general theory of living systems, a systems theory based on the concepts center, wholeness and transformation by using design patterns and pattern languages as parts of the method. There is a whole new cosmos of thinking with astonishing bandwidth: Alexander suggests a new scientific paradigm that stands in opposition to the traditional causal mechanistic paradigm of natural science - and he offers a new knowledge format as a way to enable people and support their participation in design processes. Alexander's theories have already proven useful in education, organizational development, permaculture, and in software the design patterns have even become mainstream. Many disciplines are about to follow in this development. Pattern theory allows us to change our thinking, to rethink our world and to move towards a fairer society. This leads to more participation and higher sustainability. Alexander's concepts form a mental toolbox for societal change and innovation.