How do you say in the back in French?

"In the back" is "dans le dos" or "� l'arrière" in French.
He was stabbed in the back: il a été poignardé dans le dos;
The children are in the back of the car: les enfants sont � l'arrière de la voiture.

Le dos (masc.) is the back of the body, l'arrière is the position of being 'in the back'.