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A $G$-category is a category on which a group $G$ acts. This work studies the $2$-category $G$-Cat of $G$-categories, $G$-functors (functors which commute with the action of $G$) and $G$-natural transformations (natural transformations which commute with the $G$-action). There is particular emphasis on the relationship between a $G$-category and its stable subcategory, the largest sub-$G$-category on which $G$ operates trivially. Also contained here are some very general applications of the theory to various additive $G$-categories and to $G$-topoi.