  We prove some half-space theorems for minimal surfaces in the Heisenberg
group Nil_3 and the Lie group Sol_3 endowed with their left-invariant
Riemannian metrics. If S is a properly immersed minimal surface in Nil_3 that
lies on one side of some entire minimal graph G, then S is the image of G by a
vertical translation. If S is a properly immersed minimal surface in Sol_3 that
lies on one side of a special plane, then S is another special plane.
