Interest and activity in particle astrophysics has continued to grow. It has now been 25 years since publication of the first edition. A new edition is long overdue, but nevertheless well-motivated in view of the growth of the field and several important discoveries in the interim. The discoveries include flavor oscillations in atmospheric and solar neutrinos, the cutoff of the spectrum of ultra-high-energy cosmic rays, TeV gamma rays from supernova remnants in the Galaxy and from distant active galaxies, an unexpected excess of positrons at high energy (but not of anti-protons) and, most recently, high-energy astrophysical neutrinos.

The discoveries are the result of major investments in the development of new instruments: the major underground experiments, Super-Kamiokande, SNO and Borexino; the giant air shower arrays, Auger and Telescope Array; the imaging atmospheric Cherenkov telescopes VERITAS, H.E.S.S. and MAGIC, and the Fermi Satellite; the particle spectrometers in space, PAMELA and AMS-02, along with balloon-borne detectors ATIC and CREAM; and the neutrino telescopes AMANDA and Baikal, ANTARES and IceCube.

Corresponding developments on the side of particle physics stem from the colliding beam machines at DESY, Fermilab and CERN. These have provided measurements of parton distribution functions over an unprecedented kinematic range, the discovery of the top quark and, most recently, the discovery of the Higgs boson. The LHC is now running at a center of mass energy equivalent to 1017 eV in the lab, well above the knee in the cosmic ray spectrum.

All of the discoveries mentioned have given rise to new questions that stimulate continuing interest in particle astrophysics. In writing this expanded edition, we have kept the basic structure of the first edition while adding chapters on new topics stimulated by some of these open questions. Topics of the new chapters include neutrino oscillations, propagation of ultra-high energy cosmic rays in the cosmic microwave background, sources of the highest energy cosmic rays and neutrino astronomy. The chapters on atmospheric muons and neutrinos, and those on acceleration and propagation of cosmic rays, go into greater depth and focus on new results. Most important are the two chapters on particle physics, which are completely new, and are intended to bring the latest results from high-energy physics to bear on cosmic ray physics.

We are grateful to many colleagues who, in one way or another, helped us to understand and explain the material in this book.