  A tensor -- meaning here a tensor field $\Theta$ of any type $(p,q)$ on a
manifold -- may be called integrable if it is parallel relative to some
torsion-free connection. We provide analytical and geometric characterizations
of integrability for differential $q$-forms, $q=0,1,2,n-1,n$ (in dimension
$n$), vectors, bivectors, symmetric $(2,0)$ and $(0,2)$ tensors, as well as
complex-diagonalizable and nilpotent tensors of type $(1,1)$. In most cases,
integrability is equivalent to algebraic constancy of $\Theta$ coupled with the
vanishing of one or more suitably defined Nijenhuis-type tensors, depending on
$\Theta$ via a quasilinear first-order differential operator. For
$(p,q)=(1,1)$, they include the ordinary Nijenhuis tensor.
