This monograph is a collective work. The names appear- ing on the front
cover are those of the people who worked on every chapter. But the
contributions of others were also very important: C. Risito for Chapters
I, II and IV, K. Peiffer for III, IV, VI, IX R. J. Ballieu for I and IX,
Dang Chau Phien for VI and IX, J. L. Corne for VII and VIII. The idea of
writing this book originated in a seminar held at the University of
Louvain during the academic year 1971-72. Two years later, a first draft
was completed. However, it was unsatisfactory mainly because it was ex-
ce sively abstract and lacked examples. It was then decided to write it
again, taking advantage of -some remarks of the students to whom it had
been partly addressed. The actual text is this second version. The
subject matter is stability theory in the general setting of ordinary
differential equations using what is known as Liapunov's direct or
second method. We concentrate our efforts on this method, not because we
underrate those which appear more powerful in some circumstances, but
because it is important enough, along with its modern developments, to
justify the writing of an up-to-date monograph. Also excellent books
exist concerning the other methods, as for example R. Bellman [1953]
and W. A. Coppel [1965].