Hydroxyapatite (HAp) is a bioceramic material that is the main inorganic component of hard and connective tissues (e.g., bones and teeth). Therefore efforts to develop novel biomaterials as bone substitute are currently based on HAp composites. A good interaction and adhesion between HAp and the selected polymeric matrix is required to achieve good mechanical properties. The affinity between these two phases can be improved by chemical reaction of coupling agents with the hydroxyl groups on the surface of HAp. The present review is focused to describe the main grafting processes and surface modifications carried out on HAp surfaces and the repercussions on biomineralization and osteogenesis processes.