The twelfth SLAC Summer Institute on Particle Physics was held July 23 - August 3, 1984. The main topic was "The Sixth Quark." The Institute comprised of seven days of "Summer School," followed by three days of "Topical Conference." The school portion involved both theoretical and experimental courses on the weak interactions of quarks and leptions in general, followed by specific courses on the production of top quarks, the physics of toponium, and the role of the top quark in weak interactions and CP violation. These courses were interlaced with lectures on the technology and physics of heavy quark flavor tagging by ring imaging Cherenkov counters and by solid state precision vertex detectors. The Topical Conference confirmed reports on the most recent results on heavy quark production and decay and associated theoretical eeveopments and was highlighted by the presentation of the first experimental evidence for the existence of the top quark from the UA1 experiment at the CERN SPS collider The last day of the Topical COnference was devoted to Pief-Fest, a tribute to Pief Panofsky on his retirement as director of SLAC, with policy and physics topics of particular interest to him. The 1984 Institute was attended by 454 physicists from 11 countries. Show Partial Abstract