The power struggle between the death of Isabel I of Castile and the Comunero uprising of 1520-1521 involved both dynastic rupture and a crisis of legitimacy. While Juana's titular rights as proprietary sovereign were always recognised, her husband, father and son opposed her right to govern. The thesis challenges deeply-embedded views about Juana's political indifference, while also questioning the recent, influential contention that Juana sacrificed her rights to protect dynastic interests. Juana might have suffered intermittently from mental health problems, but was a key player, and the history of the period cannot be understood without taking her queenship, and question of her right to influence government policy, fully into account. Juana saw herself, above all, as Isabel's daughter, and a Trastámara, and her successes, failures, and changing political strategies are seen in this light. Despite her notion of filial obedience, at a time when her father, Fernando II of Aragon, who had co-reigned with Isabel, remained active and ambitious to govern Castile, Juana engaged with, and greatly influenced, major events between 1505-1507. Again, in 1520, her role during the Comunero revolution, when she came to the defence not only of her son, Charles V, but, more especially, of the principle of royal authority, proved crucially significant. The thesis explores political and cultural concepts of the time to show how they were applied to the manner in which Juana was seen, such as the development of a Queen's 'party' based on the knightly ideology of honour and loyalty; the application of the notion of 'shadow' monarch to attempts to marginalise her from power in 1506-1507, and the essentially gender-based topoi of jealousy and hysteria that informed views about the last Trastámara monarch's unfitness to govern.