Abstract: 2-groups, or "categorical groups", are "higher dimensional" algebraic structures which generalize groups. In particular, they can be seen as group objects in categories, and as such they are useful for describing the symmetries of categories. I will describe 2-groups and their actions, and describe how similar generalizations of the transformation groupoids associated to group actions show up in this context. Time permitting, I will outline a motivating example for this work coming from the geometry of gerbes. Based on joint work with Roger Picken (IST, Lisbon).