With their active apostolate of preaching and teaching, Dominican friars were important promoters of Latin Christianity in the borderlands of medieval Spain and North Africa. Historians have long assumed that their efforts to convert or persecute non-Christian populations played a major role in worsening relations between Christians, Muslims and Jews in the era of crusade and reconquista. This study sheds light on the topic by setting Dominican participation in celebrated but short-lived projects such as Arabic language studia or anti-Jewish theological disputations alongside day-to-day realities of mendicant life in the medieval Crown of Aragon. From old Catalan centers like Barcelona to newly conquered Valencia and Islamic North Africa, the author shows that Dominican friars were on the whole conservative educators and disciplinarians rather than innovative missionaries - ever concerned to protect the spiritual well-being of the faithful by means of preaching, censorship and maintenance of existing barriers to interfaith communications.