**Editors: Carl M. Bender, Maarten DeKieviet, and S. P. Klevansky **

*PT*-symmetric quantum mechanics (PTQM) has become a hot area of research and investigation. Since its beginnings in 1998, there have been over 1000 published papers and more than 15 international conferences entirely devoted to this research topic. Originally, PTQM was studied at a highly mathematical level and the techniques of complex variables, asymptotics, differential equations and perturbation theory were used to understand the subtleties associated with the analytic continuation of eigenvalue problems. However, as experiments on *PT*-symmetric physical systems have been performed, a simple and beautiful physical picture has emerged, and a *PT*-symmetric system can be understood as one that has a balanced loss and gain. Furthermore, the *PT* phase transition can now be understood intuitively without resorting to sophisticated mathematics.

Research on PTQM is following two different paths: at a fundamental level, physicists are attempting to understand the underlying mathematical structure of these theories with the long-range objective of applying the techniques of PTQM to understanding some of the outstanding problems in physics today, such as the nature of the Higgs particle, the properties of dark matter, the matter–antimatter asymmetry in the universe, neutrino oscillations and the cosmological constant; at an applied level, new kinds of *PT*-synthetic materials are being developed, and the *PT *phase transition is being observed in many physical contexts, such as lasers, optical wave guides, microwave cavities, superconducting wires and electronic circuits. The purpose of this Theme Issue is to acquaint the reader with the latest developments in PTQM.

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