Descent in Buildings begins with the resolution of a major open question about the local structure of Bruhat Tits buildings The authors then put their algebraic solution into a geometric context by developing a general fixed point theory for groups acting on buildings of arbitrary type giving necessary and sufficient conditions for the residues fixed by a group to form a kind of subbuilding or form of the original building At the center of this theory is the notion of a Tits index a combinatorial version of the notion of an index in the relative theory of algebraic groups These results are combined at the end to show that every exceptional Bruhat Tits building arises as a form of a residually pseudo split Bruhat Tits building The book concludes with a display of the Tits indices associated with each of these exceptional forms This is the third and final volume of a trilogy that began with Richard Weiss The Structure of Spherical Buildings and The Structure of Affine Buildings