This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book summarizes important scientific findings regarding the aquatic effects of acidic deposition subsequent to publication of the State of Science and Technology Reports by National Acid Precipitation Assessment Program (NAPAP) in 1990. NAPAP was reauthorized under the 1990 Clean Air Act Amendments. The book provides a state of science update on advancements in acidic deposition aquatic effects research since 1990. It also provides background material on acidic deposition, the major response variables of concern, and methods for evaluating acidification. The book outlines advancements in the understanding of chronic acidification and the characteristics of sensitive systems. It summarizes the results of selected examples of whole-ecosystem experimental acidification and de-acidification studies and considers aspects of acidification modeling of aquatic effects. NAPAP developed a complex research and assessment process that involved many hundreds of scientists throughout the US, Canada, and Europe.