The book attempts to provide a consistent treatment climate variability at time scales longer than interannual. The first describes the observed decadal variability when there are sufficient observational data for analysis, identifying the major phenomena that are mainly involved. The second part contains contributions describing the present level of understanding of decadal variability. Researchers and students will find the book useful as a reference, and scientists in related disciplines (geology, biogeochemistry, paleoclimatology) will have an overview of current knowledge.