This chapter examines the role of military engineers in the defence of Portugal during the Restoration War, 1640–68. From the beginning of the war with Spain to assert its independence, Portugal’s Restoration government recognized the importance of fortifications in its defence. Given the developments in warfare and weapons and the military architecture needed to withstand them – the so-called Military Revolution (MR) of the early modern period – the existing and largely neglected medieval military architecture of Portugal was unlikely to withstand attack from its opponent, one of Europe’s leading military powers. The newly formed Conselho de Guerra recognized the need for experts to survey, design, adapt, and build the new style of fortifications. This chapter explores their provenance, recruitment, training, experience, and deployment and the growing debate over the use – and dangers – of foreign instead of native-born military engineers. It draws comparisons with the military engineers deployed elsewhere in early modern Europe.