Papers from the symposium MEGA-90, held in Livorno, Italy, April 1990, on effective methods and complexity issues in commutative algebra, projective geometry, real geometry, and algebraic number theory; algebraic geometric methods in algebraic computing; and contributions in related fields (computational aspects of group theory, differential algebra and geometry, algebraic and differential topology, etc.). A number of the papers are in French. No index. Annotation copyright Book News, Inc. Portland, Or.