 This review discusses the effects of various ions and surface states on the biocompatibility of hydroxyapatite, HA, a calcium phosphate compound found in biological hard tissues like bones and teeth. HA's chemical composition and crystal structure can be altered by adding different elements or ions, which can affect its surface properties and thus its biocompatibility. These surface properties include the presence of hydration layers and non-apatite layers, which are important for controlling protein adsorption and cell adhesion. By analyzing these surface properties, researchers hope to gain insight into how HA can be used to promote bone formation and regeneration. This article was authored by Kezito Sugimoto, Yanni Zhou, Tanya Guadalupe, Penaflor Galindo, and others.