This book is a modified version of a course given at Orsay in 1976-77.
The aim of the course was to give a comparatively elementary proof of
the Enriques classification of complex algebraic surfaces, accessible to a
student familiar with the basic language of algebraic geometry (divisors,
differential forms, ...) as well as sheaf cohomology. I have, however,
preferred to assume along the way various hard theorems from algebraic
geometry, rather than resort to complicated and artificial proofs