This book provides foresters with the necessary information to understand the performance of spruce seedlings after being planted on a reforestation site. It was written for an anticipated audience of university students who are taking a regeneration silviculture class, plus foresters and researchers who work with spruce species within forest regeneration programs throughout the world. The book was designed so that the reader can develop an initial understanding of primary physiological processes of spruce seedlings and the importance of these processes when making silvicultural decisions. The scientific discipline of ecophysiology examines the physiological and morphological processes of plants in response to the surrounding environment. The primary focus of this book is the physiological processes of northern spruce species at the whole plant level in response to the surrounding environment. The following fundamental physiological processes are discussed: water relations, gas exchange, mineral nutrition, freezing tolerance, dormancy, and morphological development. Examples of how genetic variation can affect the ecophysiological response of spruce species are also presented. The book briefly examines major components of the seedling environment on reforestation sites and how these components influence seedling responses.