Unlike most theologians of his age, Martin Bucer had a wide range of vision with respect to European affairs: In addition to his contacts within Alsace and Germany, he established relations with almost every country on the Continent. It was his ecumenical attitude that always led him to mediate between the parties in the religious battles of his time. His deep commitment and his objective to reach an agreement can be traced in all his activities, works and letters. As Bucer did not found a religious denomination himself, his theological and historical importance has been underestimated for a long time. In addition his handwriting is hard to decipher, which makes it difficult to deal with his works, especially with his letters. Bucer's letters (BCor) have been published in chronological order as part of the "Opera omnia" since 1979 (Leiden, Brill, I: 1979; II: 1989; III: 1995; IV: 2000). Since the editor, Jean Rott (Strasbourg), died Bucer's correspondence has been edited in Erlangen. This academic edition of source material will provide future research with a broad basis for significant aspects of Reformation history about which very little is known.