Felix Hausdorff is one of the outstanding mathematicians of the first half of the 20th century. He left behind an unusually rich corpus of scientific manuscripts. His complete works are to be published in 9 volumes, each with detailed commentary. The present Volume II contains Hausdorff's most important work, "Foundations of Set Theory." This book is considered a classic of mathematical literature and has had a significant impact on the development of mathematics in the 20th century. Therefore, it seemed appropriate to include extensive commentary. These comments primarily address the significant original contributions that Hausdorff made to topology, general set theory, and descriptive set theory in the "Foundations." In particular, an effort is made to place Hausdorff's achievements within the historical development.