We study the properties of level-zero modules over quantized affine
algebras. The proof of the conjecture on the cyclicity of tensor
products by T. Akasaka and the author is given. Several properties of
modules generated by extremal vectors are proved. The weights of a
module generated by an extremal vector are contained in the convex
hull of the Weyl group orbit of the extremal weight. The universal
extremal weight module with level-zero fundamental weight as an
extremal weight is irreducible, and it is isomorphic to the
affinization of an irreducible finite-dimensional module.