Our understanding of coupled cluster theory as described in this chapter has been cultivated by conversations and collaborations with numerous colleagues over the past several years. In particular, we would like to acknowledge many fruitful interactions with Professor John F. Stanton of the University of Texas, Dr. Timothy J. Lee of NASA Ames Research Center, Professor Marcel Nooijen of Princeton University, and Professor Rodney J. Bartlett of the University of Florida. We thank Sullivan Beck of the University of Florida for the use of his diag program used to typeset the many diagrams in the chapter. We also thank Rollin A. King and Matthew L. Leininger of the University of Georgia and Dr. C. David Sherrill of the University of California at Berkeley for their critical reading of the manuscript. Finally, TDC wishes to express his sincere appreciation to Emily A. Crawford for her work in the development of the contract LATEX package for typesetting complicated Wick's theorem expressions, and for her patience and support during the preparation of the manuscript.