The advance of scienti?c thought in ways resembles biological and geologic transformation: long periods of gradual change punctuated by episodes of radical upheaval. Twentieth century physics witnessed at least three major shifts relativity, quantum mechanics and chaos theory as well many lesser ones. Now, st early in the 21 , another shift appears imminent, this one involving the second law of thermodynamics. Over the last 20 years the absolute status of the second law has come under increased scrutiny, more than during any other period its 180-year history. Since the early 1980s, roughly 50 papers representing over 20 challenges have appeared in the refereed scienti?c literature. In July 2002, the ?rst conference on its status was convened at the University of San Diego, attended by 120 researchers from 25 countries (QLSL2002) [1]. In 2003, the second edition of Le?s and Rexs classic anthology on Maxwell demons appeared [2], further raising interest in this emerging ?eld. In 2004, the mainstream scienti?c journal Entropy published a special edition devoted to second law challenges [3]. And, in July 2004, an echo of QLSL2002 was held in Prague, Czech Republic [4]. Modern second law challenges began in the early 1980s with the theoretical proposals of Gordon and Denur. Starting in the mid-1990s, several proposals for experimentally testable challenges were advanced by Sheehan, et al. By the late 1990s and early 2000s, a rapid succession of theoretical quantum mechanical ? challenges were being advanced by C apek, et al.

Entropy and the Second Law.- Challenges (18701980).- Modern Quantum Challenges: Theory.- Low-Temperature Experiments and Proposals.- Modern Classical Challenges.- Gravitational Challenges.- Chemical Nonequilibrium Steady States.- Plasma Paradoxes.- MEMS/NEMS Devices.- Special Topics.