A number of important particle astrophysics and cosmic ray experiments can be carried out at a lunar base which are impossible or impractical on the earth or in earth orbit:i) A measurement of the cosmic ray spectrum and composition in the 1014to 1016eV/nucleus region is crucial for the understanding of high energy cosmic ray sources and energetic particle acceleration mechanisms, but requires detectors too large and massive to be lifted into earth orbit. A calorimeter on the moon could provide the large area and detector mass required by using lunar soil for the bulk of the detector mass, and lifting only the active detector components and support structure from the earth. A lunar base calorimeter may be the only feasible way to do this important experiment.ii) Observation of heavy neutrinos or the products of WIMP annihilations would be of fundamental importance to particle physics and cosmology.Such observations are probably impossible on earth (where the terrestrial atmosphere produces too much background). At a lunar site, the rock itself acts as a target for upward‐moving neutrinos; these then produce secondary muons which are observed as they continue moving upward through the detector.iii) Ultraheavy cosmic ray, isotope, and antimatter studies will shed light on nucleosynthesis processes and the properties of the interstellar medium, and will require large area detectors in a low magnetic field.iv) And observaition of high energy gamma ray sources in the currently unexplored 5–200 GeV window may be able to take advantage of the presence of a (sufficiently large) lunar base water supply at very little additional cost.