Heme-containing proteins are one of the most structurally and functionally diverse groups of proteins in nature. Central to our understanding of their function is an appreciation of the fundamental inorganic and physical properties of the heme prosthetic group itself. Many spectroscopic techniques have been used to probe heme proteins but these alone often cannot reveal all of the key information required. Many exogeneous heme-iron ligands have been shown to be highly sensitive to the electronic and physical properties of protein-bound heme groups. Such ligands, used in combination with spectroscopic and/or crystallographic analyses, have proved to be particularly useful in probing not only the heme prosthetic group itself but also the surrounding structure and dynamics of the protein active-site. In this perspective, we introduce five diverse families of heme-proteins and discuss how the use of heme-coordinating ligands has provided immensely important information about the physical and structural properties of each heme-protein family.