  In this paper we study the effect of a homoclinic tangency in the variation
of the topological entropy.
  We prove that a diffeomorphism with a homoclinic tangency associated to a
basic hyperbolic set with maximal entropy is a point of entropy variation in
the $C^{\infty}$-topology. We also prove results about variation of entropy in
other topologies and when the tangency does not correspond to a basic set with
maximal entropy.
  We also show an example of discontinuity of the entropy among $C^{\infty}$
diffeomorphisms of three dimensional manifolds.
