The wave of tax revolts that swept across the country in 1637 marked one of the most significant expressions of civil unrest in Portugal during the Spanish Habsburg rule. Preceded by some minor incidents in northern Portugal in 1635 and 1636, the real revolt broke out in the city of Évora in August 1637, and quickly escalated into open defiance of royal authority, including attacks on tax collectors and local magistrates, the occupation of public buildings, and the suspension of regular government. From Évora, the unrest spread rapidly throughout the Alentejo and beyond (in Algarve, Ribatejo, Estremadura, Minho and Trás-os-Montes), with dozens of towns and villages experiencing similar outbreaks of protest—many of them spontaneous, others clearly inspired by the events in Évora. The movement brought together a diverse cross-section of society, including artisans, merchants, clergy, and rural inhabitants. Although often sparked by local grievances, these disturbances were largely driven by widespread discontent with the fiscal pressures imposed by the Spanish crown, which had increased taxes (mostly the ‘real de água’ and the ‘sisas’) allegedly to fund military campaigns, particularly those related to the war with the Dutch in Brazil. Though ultimately suppressed by royal troops sent from Spain, the tax riots of 1637 deeply alarmed the Spanish administration and revealed the fragility of its rule in Portugal. NB: The local events are addressed in separate entries within this Encyclopaedia.