This paper describes the topic of Byzantine perception, both of memebers of the upper and lowe social classes, of the upcoming participants of the First Crusade, but also of the very idea of the crusade. The paper first presents the internal state of the Eastern Roman Empire and the overall state in the Latin world, i.e. in Germany, France and Italy. Furthermore, the work follows the arrival of the crusaders in the territory under the control of Byzantine emperor and describes the meeting of those two different cultures. The second part of the paper deals with the period of the beginnig and during the war part of the Firts Crusade, i.e. from the initial cooperation between the Byzantines and the Latins until the break of the alliance and open hostility between Bohemund and Alexius I. Komnenos. At the end of the paper, further repercussions on diplomatic relations of Christian East and West in the 12th century are presented.