  Let $G$ be a simple algebraic group over an algebraically closed field $K$ of
characteristic $p\geqslant 0$, let $H$ be a proper closed subgroup of $G$ and
let $V$ be a nontrivial irreducible $KG$-module, which is $p$-restricted,
tensor indecomposable and rational. Assume that the restriction of $V$ to $H$
is irreducible. In this paper, we study the triples $(G,H,V)$ of this form when
$G$ is a classical group and $H$ is positive-dimensional. Combined with earlier
work of Dynkin, Seitz, Testerman and others, our main theorem reduces the
problem of classifying the triples $(G,H,V)$ to the case where $G$ is an
orthogonal group, $V$ is a spin module and $H$ normalizes an orthogonal
decomposition of the natural $KG$-module.
