We present the results of a study of the chemical and magnetic structures of a series of holmium-yttrium superlattices and a 5000 angstrom film of holmium, all grown by molecular-beam epitaxy. By combining the results of high-resolution x-ray diffraction with detailed modeling, we show that the superlattices have high crystallographic integrity: the structural coherence length parallel to the growth direction is typically almost-equal-to 2000 angstrom, while the interfaces between the two elements are well defined and extend over approximately four lattice planes. The magnetic structures were determined using neutron-scattering techniques. The moments on the Ho3+ ions in the superlattices form a basal-plane helix. From an analysis of the superlattice structure factors of the primary magnetic satellites, we are able to determine separately the contributions made by the holmium and yttrium to the total helical turn angle per bilayer. It is found that the effective turn angle per atomic plane in the yttrium, which has a value of approximately 50-degrees, is independent of both temperature and the number of yttrium or holmium planes. The turn angle in the holmium blocks changes with temperature, and always has a value that is greater than in bulk holmium. The variation in the turn angle with temperature depends on the length of the holmium block, but is largely independent of the thickness of the yttrium block. At low temperatures, the (1/6)c* phase found in bulk holmium is suppressed. The observation of high-order magnetic satellites indicates that the moments instead form long-period, commensurate spin-slip structures. The results are discussed in terms of the strain present in these samples.