Anders Kock; The stack quotient of a groupoid ia available at the address http://www.ml.kva.se/preprints/meta/KockMon_Jun_11_10_57_41.rdf.html Summary: Making explicit in which sense the stack of principal G-bundles is the quotient of the groupoid G. A main tool is an equivalence between descent data, and lift of the groupoid G into a groupoid in a fibration over G. Anders K. (Note: my permanent e-mail address is unchanged, kock@imf.au.dk)