This edition contains four new sections on the following topics： the BDDC domain decomposition preconditioner （Section 7.8）, a convergent adaptive algorithm （Section 9.5）, interior penalty methods （Section 10.5） and Poincare-Friedrichs inequalities for piecewise Wp1 functions （Section 10.6）.We have made improvements throughout the text, many of which were suggested by colleagues, to whom we are grateful. New exercises have been added and the list of references has also been expanded and updated.