  The disk complex of a surface in a 3-manifold is used to define its {\it
topological index}. Surfaces with well-defined topological index are shown to
generalize well-known classes, such as incompressible, strongly irreducible,
and critical surfaces. The main result is that one may always isotope a surface
$H$ with topological index $n$ to meet an incompressible surface $F$ so that
the sum of the indices of the components of $H \setminus N(F)$ is at most $n$.
This theorem and its corollaries generalize many known results about surfaces
in 3-manifolds, and often provides more efficient proofs. The paper concludes
with a list of questions and conjectures, including a natural generalization of
Hempel's {\it distance} to surfaces with topological index $\ge 2$.
