Over the past decade or so, great progress has been made in two diverse areas of physics—cosmology and elementary-particle physics. In spite of the obvious differences of the two fields (figure 1), each has begun to illuminate the other, making interdisciplinary work involving them not only possible but even exciting. Thus, for example, the cosmological abundance of helium-4 fixes an upper limit of 8 on the number of quark varieties (“flavors”) in models that have a symmetry between quarks and leptons. And developments in the grand unified theories of elementary processes may resolve the puzzle of why there are roughly a billion photons for every baryon in the universe. As our knowledge of the fundamental particles and their interactions increases, and as our determination of cosmological observables improves (or new observables are discovered) the close relationship of these two disciplines promises to continue to be an exciting one.