This volume consists of papers developed from a joint ACE/ISSI symposium on the "Composition of Matter" honoring the occasion of Johannes Geiss's 80th birthdayThe purpose of the symposium was to explore new insights into the composition of solar-system and galactic matter, and fractionation processes affecting samples of this matter, that have been brought about by recent space missions, ground-based studies, and theoretical advances. It was convened by G. M. Mason (chair), P. Bochsler, J. J. Connell, G. Gloeckler, M. H. Israel, R. A. Mewaldt, R. von Steiger, M. E. Wiedenbeck, and T. H. Zurbuchen. The convenors compiled a program of overview, invited, contributed, and poster presentations that was organized into five sessions: Linking primordial to solar composition, Planetary samples, Solar sources and fractionation processes, Interstellar gas and Cosmic rays.The structure of the volume at hand largely follows the structure of the symposium. The editors are happy that it includes almost all of the overview and invited papers and many of the contributed ones.