The parathyroid glands are small (<50 mg) endocrine glands located on the posterior aspect of the thyroid. The primary function of the glands is to produce and secrete parathyroid hormone (PTH), which regulates calcium homeostasis in the circulation. PTH deficiency (hypoparathyroidism) or excess (hyperparathyroidism) may be caused by mutations in genes required for development of the parathyroid gland or physiological function as well as other causes. As knowledge of the human genome increases, advances in diagnostic and treatment techniques for parathyroid-related disorders will undoubtedly involve a thorough understanding of the key molecular processes guiding parathyroid development and function. This chapter provides a review of these events, as currently understood, as well as their potential clinical relevance.