P. J. Higgins Originally published by: Van Nostrand Reinhold, 1971 Keywords: groupoids, subgroup theorems, covering morphisms, fundamental group, homology of groupoids 2000 MSC: 20L05, 18-01, 20E07, 20J05, 18B40, 18G30, 55Q05 Republished in: Reprints in Theory and Applications of Categories, No. 7 (2005) pp 1-195 http://www.tac.mta.ca/tac/reprints/articles/7/tr7.pdf Additional page set-ups - see N