This monograph presents in detail the reduction method for studying the
unification of fundamental actions. The mathematical (differential
geometrical) methods make extensive use of Lie Groups and the concept of
homogeneous spaces. The main topic of the book is the dimensional
reduction of pure Yang-Mills theories. A rather complete analysis of the
structure of the scalar field potential is given and a general procedure
for solving the equations of spontaneous compactification within
Einstein-Yang-Mills systems is presented. The authors also discuss
gravity and theories with fermions included and they review attempts to
construct realistic models. The book presents the basic ideas and the
calculations in detail and should be of interest to researchers and
graduate students in mathematical physics.