  A recent problem in dynamics is to determinate whether an attractor $\Lambda$
of a $C^r$ flow $X$ is $C^r$ robust transitive or not. By {\em attractor} we
mean a transitive set to which all positive orbits close to it converge. An
attractor is $C^r$ robust transitive (or {\em $C^r$ robust} for short) if it
exhibits a neighborhood $U$ such that the set $\cap_{t>0}Y_t(U)$ is transitive
for every flow $Y$ $C^r$ close to $X$. We give sufficient conditions for
robustness of attractors based on the following definitions. An attractor is
{\em singular-hyperbolic} if it has singularities (all hyperbolic) and is
partially hyperbolic with volume expanding central direction \cite{MPP}. An
attractor is {\em $C^r$ critically-robust} if it exhibits a neighborhood $U$
such that $\cap_{t>0}Y_t(U)$ is in the closure of the closed orbits is every
flow $Y$ $C^r$ close to $X$. We show that on compact 3-manifolds all $C^r$
critically-robust singular-hyperbolic attractors with only one singularity are
$C^r$ robust.
