This dissertation is a comparative study of Urraca of León-Castilla (r. 1109–1126), Melisende of Jerusalem (r. 1131–1153–d. 1161), and the Empress Matilda of England and Normandy (b. 1102–d. 1167). Despite the vast research on aristocratic heiresses and queens consorts, a comparative study of royal heiresses as rulers in their own right does not exist. The few studies that focus on royal heiresses examine individual royal women or are region-specific studies. However, by studying royal heiresses comparatively, greater insight can be gained regarding the challenges women faced in their attempt to gain the throne, the methods they employed to keep power, and the unique variations of rulership that are specific to each queen regnant. In general, medieval society expected royal power would be held by men, but in the absence of a male heir, women, on occasion, held royal office. This study observes how royal heiresses could mostly, but not always, overcome the limitations of their gender to establish a rule in their respective kingdoms. This thesis explores aspects of rulership over five chapters, aimed at understanding how a royal heiress might succeed or fail to gain the throne, keep the throne, and preserve it for future generations. Through the use of a comparative methodology, this thesis provides a fresh discussion of royal heiresses as rulers. It shows that royal heiresses faced different obstacles to their rule than their aristocratic counterparts and, that because of their royal status, they were able to overcome complications that aristocratic heiresses could not. Demonstrations of female power were, in many cases, approved of at the royal level but were condemned at the aristocratic level, as was the case for Melisende of Jerusalem and her younger sister, Alice of Antioch (c. 1110–1136). Studying Urraca of León-Castilla, Melisende of Jerusalem, and the Empress Matilda side-by-side, this thesis also establishes the individual pitfalls of female rulership and identifies the methods each aspiring queen regnant utilized in order to overcome them.