This volume collects six related articles. The first is the notes (written by J.S. Milne) of a major
part of the seminar "Periodes des Integrales Abeliennes" given
by P. Deligne at I.H.E.S., 1978-79.
The second article was written for this volume (by P.
Deligne and J.S. Milne) and is largely based on: N Saavedra
Rivano, Categories tannakiennes, Lecture Notes in Math. 265,
Springer, Heidelberg 1972.
The third article is a slight expansion of part of:
J.S. Milne and Kuang-yen Shih, Shimura varieties: conjugates
and the action of complex conjugation 154 pp. (Unpublished
manuscript, October 1979).
The fourth article is based on a letter from P. Deligne
to R. Langlands, dated 10th April, 1979, and was revised and
completed (by Deligne) in July, 1981.
The fifth article is a slight revision of another section
of the manuscript of Milne and Shih referred to above.
The sixth article, by A. Ogus, dates from July, 1980.