Parathyroid hormone (PTH) and PTH-related peptide (PTHrP), along with other calciotropic hormones, play critical roles in calcium homeostasis and bone biology. This chapter provides an overview on PTH hormone and calcium homeostasis. PTHrP is produced as a paracrine/autocrine factor in many different adult and fetal tissues and has, unlike PTH, multiple functions. PTH is secreted by the parathyroid glands and is the critical regulator of blood calcium concentration in all-terrestrial vertebrate species. PTHrP is a slightly larger molecule than PTH. To ensure a multitude of essential cellular functions, the extracellular concentration of calcium is maintained within narrow limits vertebrates, calcium is necessary for adequate mineralization of the skeleton, which provides mechanical support and protection for internal organs and acts as levers for the various muscle groups involved in locomotion. The PTH1R gene is expressed in a large variety of fetal and adult tissues, but the receptor is most abundant in kidney and bone, where it mediates the endocrine actions of PTH in mineral ion homeostasis, and in the metaphyseal growth plate, where it mediates the autocrine/paracrine actions of locally synthesized PTHrP.