Why study a small lake in the White Mountains of north-central New Hamp­ shire? Better yet, why write a book about those studies? We wrestled with such questions for years, and the answers lie in the overall ecosystem focus and approach we have taken. At the same time that we studied Mirror Lake, numerous and comprehensive studies were done of the surrounding terres­ trial and stream ecosystems of the Hubbard Brook Valley. These associated studies complemented those done in the lake and provided unique informa­ tion about air-land-water linkages in the Valley. Many of the studies con­ ducted in Mirror Lake were about organisms or about the processes carried out by these individual organisms or communities. Nevertheless, an abiding objective always was to determine the significance that these individuals and individual processes had to the overall structure, metabolism and biogeo­ chemistry of the Mirror Lake ecosystem. To wit, what is the significance for the ecosystem? In some cases the ecosystem role was clear, for many it was not. But the hope is that our attempts to unravel and understand the whole will be informative and, more importantly, that it will stimulate others to study the holistic and functional relationships in entire landscapes. The book is more than a case history of a single lake. We have written it not only for the serious student of lakes, but also for those interested in ecosystems and their interactions.