This open access book serves as a reference for the key elements and their significance of Klaus Hasselmann''s work on climate science and on ocean wave research, all based on a rigorous and deeply physical thinking. It summarizes the original articles (mostly from the 1970 and 1980s; some of which are hard to find nowadays) and brings them in a present-day context. From 1975 until 2000, he was (founding) Director of the Max Planck Institute of Meteorology, which he made to one of the world-leading academic institutions. He first made the issue of anthropogenic climate change accessible to analysis and prediction and later transformed climate science into a significant factor in forming public policy. The book is written by co-workers and colleagues of Klaus Hasselmann, whomany under his immediate supervisionjoined him in this effort. With this background, they present the key achievements and assess the significance of these for the present state of knowledge and scientific practice.