The early expansion of Islam led in time to widespread conversions of Christians in conquered territories. In the later eleventh century, however, western Christendom was in turn launching offensives against Islam on several fronts. Territorial gains were made in various Mediterranean regions and, although by the end of the thirteenth century the Holy Land had been lost again, Sicily remained in Christian hands, and in the second half of the thirteenth century in the Iberian peninsula only Granada remained under Muslim control: the whole peninsula was under Christian rule before the end of the fifteenth century. This expansion was accompanied, especially in the thirteenth century, by attempts to convert Muslims and other non-Christians. Yet in the period from the late eleventh until the later fifteenth century some western Christians converted to Islam. The purpose of the present paper is to consider the situations that prompted the adoption of Islam, and the reasons for such conversions, although the evidence is usually insufficient to indicate exactly why a particular Christian became a Muslim: the preconceived ideas voiced in western sources about forced conversions can be misleading and, although a crude distinction might be made between conversions from conviction and those based on worldly considerations, motives did not necessarily always fit neatly into just one of these two categories. But obviously not all converts would have had an equal understanding of the nature of Islamic beliefs and practices. The response of western ecclesiastical and secular authorities to renegades will also be considered. Further conversions of Christian peoples who had already for centuries been living under Muslim rule will not be examined, but only the adoption of Islam by those whose origins lay in western Christian countries or who were normally resident in these, and by westerners whose lands were newly conquered by Muslim powers after the eleventh century; and the focus will be mainly, though not exclusively, on the crusader states and the Iberian peninsula.