  This paper is an addition to the book [54] on Compact projective planes. Such
planes, if connected and finite-dimensional, have a point space of topological
dimension 2, 4, 8, or 16, the classical example in the last case being the
projective closure of the affine plane over the octonion algebra. The final
result in the book (which was published 20 years ago) is a complete description
of all planes admitting an automorphism group of dimension at least 40. Newer
results on 8-dimensional planes have been collected in [52]. Here, we present a
classification of 16-dimensional planes with a group of dimension at least 35,
provided the group does not fix exactly one flag, and we prove several further
theorems, among them criteria for a connected group of automorphisms to be a
Lie group. My sincere thanks are due to Hermann H\"ahl for many fruitful
discussions.
