Therapeutic radiation for cancer of the head and neck damages salivary glands that are situated between the radiation source and the target tumor and its metastases. With moderate-to-high radiation exposure, salivary glands are devastated, and regeneration is limited. The resulting severe reduction in saliva has detrimental effects on teeth and oral mucosa. The purpose of this review is to describe some of the salient histologic features of salivary gland structures and cells, how these are functionally related to salivary production, and thus how radiation-induced loss and functional impairment of each type of structure may contribute to reduced quantity and quality of saliva.