We present the results of a study of the chemical and magnetic structures of Ho/Y and Ho/Lu superlattices, all grown by molecular beam epitaxy. By combining the results of high-resolution X-ray diffraction with detailed modelling we show that the superlattices have high crystallographic integrity: the average structural coherence length in the growth direction is approximately 2000 angstrom, while the interfaces between the two elements are well defined, extending over approximately four lattice planes. The magnetic structures were determined using neutron scattering techniques. In the case of the Ho/Y superlattices, the Ho3+ moments form a basal-plane helix at all temperatures in the ordered phase, with an apparent suppression of the low-temperature cone phase. Instead spin-slip structures are formed, including some not observed in bulk Ho. For the Ho/Lu superlattices the magnetic structure was found to depend on n(Ho), the number of atomic planes in the Ho block. If n(Ho) > 20, then the Ho3+ Moments form a basal-plane helix down to the lowest temperatures. For n(Ho) less-than-or-equal-to 20, the Ho3+ moments undergo a phase transition at low temperatures from a helix to a state where they are ferromagnetically coupled within one block. In this phase, successive Ho blocks are antiferromagnetically coupled, and there is a small component of the moment along the c axis.