This study offers a new perspective on the question of how the Upper German Anabaptist traditions of the 16th and 17th centuries became part of the Mennonite denominational family. In modern scholarship, it is a commonly accepted usage to apply the group name "Swiss Brethren" to early Swiss Anabaptism starting with the circle around Conrad Grebel and Felix Mantz in Zurich who introduced the practice of believers' baptism in January, 1525. This usage is relatively recent. There are no contemporary Swiss sources which employ the group name "Swiss Brethren" in connection with the nascent Zurich Anabaptists or the early dissemination of Anabaptism in Switzerland. It was not before 1538/39 that the name "Swiss Brethren" first appeared in the sources, but referring to a group in Moravia and southwest Germany rather than in Switzerland. The modern usage of the name as referring to the Zurich and early Swiss Anabaptists goes back to Ludwig Keller (1885). It was introduced to Mennonite historiography by John Horsch and effectively popularized by Harold Bender and John Howard Yoder. Based on a detailed analysis and contextualization of 141 sources which (possibly) bear evidence of the group name "Swiss Brethren", dating from the 1530s to c.1618, the present study suggests to abandon the commonly accepted identification of the "Swiss Brethren" with the Anabaptist groups on Swiss territory or with a specifically Swiss tradition within Upper German Anabaptism. Instead, the bits and pieces of information contained in the analyzed sources adumbrate the picture of an expanding underground denomination, its leadership and its organizational structures which included periodical general synods and regional conferences of ministers and elders. While it is highly probable (but scantily documented) that this clandestine Anabaptist church included congregations in various regions of Switzerland from the beginnings, its geographical focuses seem to have been Moravia, Württemberg, the Palatinate and the Alsace. From 1555 on, part of the Lower Rhenish Melchiorite-Mennonite Anabaptist congregations joined the Swiss Brethren communion, with Cologne and Aachen as important urban centers and even with some outposts in the Netherlands.