 Bartlett and his coworkers were the first to formulate and implement coupled cluster theory with all single and double excitation operators (CCSD) in 1982, followed by triple (CCSDT) in 1987, quadruple (CCSDTQ), and even quintuple (CCSDTQP) excitation operators, and also many-body perturbation methods up to the sixth order.He developed a version of Feynman diagrams that both expedited the derivation of the equations and helped to visualize the physics of electron correlation.He also promoted the concept of size extensivity for many-body theory that scales properly with the number of particles, now viewed as an essential element of sound quantum chemistry approximations.