  Geometric approach to classical and exceptional groups of Lie type has been
quite successful and has led to the deveopment of the concept of buildings and
polar spaces. The latter have been characterized by simple systems of axioms
with a combinatorial-geometric flavour. Similar to buildings geometries can be
associated with finite sporadic simple groups (FSSGs). However, most of the
known characterizations of such geometries for FSSGs require additional
assumptions of a group-theoretic nature. One aim of this thesis is to present
characterizations of geometries for FSSGs J_2, Suz, McL, Co_3, Fi(22), Fi(23),
Fi(24) and He, which are in the same spirit as the characterizations of
buildings and polar spaces mentioned above, in particular without any
assumption on the automorphism groups of the geometries. A by-product of these
results for J_2, Suz and He is a proof that certain presentations for those
groups are faithful. Most of this work may be viewed as a contribution to the
theory of graphs with prescribed neighbourhood. The result on graphs of
(+)-points of GF(3)-orthogonal spaces, which is also used for characterization
of geometries for Fi(22), Fi(23) and Fi(24), may be considered as a
generalization of a well-known theorem on locally co-triangular graphs.
Hyperovals of polar spaces are natural generalizations of hyperovals of
projective planes of even order and play an important role in investigations of
extensions of polar spaces. As a by-product new extended generalized
quadrangles were found as hyperovals of the polar spaces Q_5^+(4) and H_5(4).
