This book contains the proceedings of the Fourth Meeting on CPT and Lorentz Symmetry, held at Indiana University in Bloomington on August 8-11, 2007. The Meeting focused on experimental tests of these fundamental symmetries and on important theoretical issues, including scenarios for possible relativity violations. Experimental subjects covered include: astrophysical observations, clock-comparison measurements, cosmological birefringence, electromagnetic resonant cavities, gravitational tests, matter interferometry, muon behavior, neutrino oscillations, oscillations and decays of neutral mesons, particle-antiparticle comparisons, post-Newtonian gravity, space-based missions, spectroscopy of hydrogen and antihydrogen, and spin-polarized matter. Theoretical topics covered include: physical effects at the level of the Standard Model, General Relativity, and beyond; the possible origins and mechanisms for Lorentz and CPT violations; and associated issues in field theory, particle physics, gravity, and string theory. Contributors consist of the leading experts in this very active research field. Sample Chapter(s). Improved Tests of Lorentz and Cpt Symmetry Using Noble-Gas Masers (447 KB). Contents: Improved Tests of Lorentz and CPT Symmetry using Noble-Gas Masers (A Glenday et al.); Rotating Experiments to Test Lorentz Invariance in the Photon Sector (M E Tobar et al.); Perspectives on Lorentz and CPT Violation (V A Kosteleckcents); Lorentz Violation in a Diffeomorphism-Invariant Theory (R Jackiw); Studies of CPT Symmetry with ASACUSA (R S Hayano); Torsion Balance Test of Preferred-Frame and Weak Coupling to Polarized Electrons (B R Heckel et al.); Seeking a Solution of the Pioneer Anomaly (M M Nieto & J D Anderson); Preliminary Results from a Test of CPT and Lorentz Symmetry using a K- 3 He Co-magnetometer (T W Kornack et al.); Ives-Stilwell for the New Millennium (M A Hohensee et al.); Data Tables for Lorentz and CPT Violation (V A Kosteleckcents & N Russell); and other papers. Readership: Theoretical and experimental physicists with interests in relativity, spacetime symmetries, and underlying unified t