The story of this book began in the late 1960s, when Prof. Buarque Borges
invited me to give a graduate course at the Aeronautics Institute of Technology.
The course was to deal with the perturbation theories used in Celestial
Mechanics, but they should be presented in a universal way, so as to be understandable
by investigators and students from related fields of science. This
hint marked the rest of the story. The course evolved and for the past 30 years
was taught almost yearly at the University of S˜ao Paulo and, occasionally, in
visited institutions abroad. A long visit of Prof. Gen-Ichiro Hori to the University
of S˜ao Paulo was the occasion for many illuminating discussions on
the subject.