This disclosure features chemical entities (e.g., a compound or a pharmaceutically acceptable salt and/or hydrate and/or prodrug of the compound) that modulate (e.g., agonize) the apelin receptor (also referred to herein as the APJ receptor; gene symbol APLNR). This disclosure also features compositions containing the same as well as other methods of using and making the same. The chemical entities are useful, e.g., for treating a subject (e.g., a human) having a disease, disorder, or condition in which a decrease in APJ receptor activity (e.g., repressed or impaired APJ receptor signaling; e.g., repressed or impaired apelin-APJ receptor signaling) or downregulation of endogenous apelin contributes to the pathology and/or symptoms and/or progression of the disease, disorder, or condition. Non-limiting examples of such diseases, disorders, or conditions include: (i) cardiovascular disease; (ii) metabolic disorders; (iii) diseases, disorders, and conditions associated with vascular pathology; and (iv) organ failure; (v) diseases, disorders, and conditions associated with infections (e.g., microbial infections); and (vi) diseases, disorders, or conditions that are sequela or comorbid with any of the foregoing or any disclosed herein. More particular non-limiting examples of such diseases, disorders, or conditions include pulmonary hypertension (e.g., PAH); heart failure; type II diabetes; renal failure; sepsis; and systemic hypertension.