The properties of Ho-Sc alloys and superlattices grown by molecular-beam epitaxy have been investigated using x-ray and neutron-diffraction techniques. Structural studies reveal that the alloy samples have different a lattice parameters for the Sc-seed layer and the Ho:Sc alloy grown on top of the seed layer; while the superlattices have different a lattice parameters for the Sc seed, and for both the Ho and Sc in the superlattice layers. The structural characteristics are related to the large lattice mismatches (of the order 7%) between the constituent elements. The magnetic moments in the alloys form a basal-plane helix at all temperatures, with distortions of the helical arrangement for samples with the highest Ho concentrations. The dependences of the Neel temperature, T-N and the helical wave vector upon both temperature and concentration are compared with those of other alloy systems. It is found that a good description of the dependence of T-N upon concentration is given by a virtual-crystal model where the peak in the conduction-electron susceptibility varies linearly between that of the pure constituents. In the superlattices, the moments also form a basal-plane helix at T-N. In this helical phase, some samples exhibit a short-range coherence of an antiferromagnetic coupling between adjacent Ho blocks. For one superlattice, there is a low-temperature transition to a ferromagnetic phase, in which moments are ferromagnetically aligned within Ho blocks, and coupled antiferromagnetically between adjacent Ho blocks. The contrast with systems which have Y or Lu as the nonmagnetic element is discussed in terms of the structural properties of the samples, band-structure calculations, and the possible influence of dipolar forces.