Sommerfeld Theory Colloquium, Three-dimensional Einstein gravity has no local dynamical degree of

freedom. Yet, it is far from being trivial when the cosmological constant is negative. (i) It admits black hole solutions. (ii) It easily allows for consistently interacting and tractable higher-spin extensions.

(iii) It possesses remarkable asymptotic properties at infi�nity where

an infi�nite-dimensional symmetry algebra emerges. These features

make three-dimensional gravity a perfect "theoretical laboratory" in which to explore the conceptual issues related to (i), (ii) and (iii) in

a simpler context. The talk will not only discuss three-dimensional

gravity assuming no previous knowledge on the subject, but will also

provide access to recent work where new results on points (i), (ii)

and (iii) are developed.