  We define a derived version of Mazur's Galois deformation ring. It is a
pro-simplicial ring $\mathcal{R}$ classifying deformations of a fixed Galois
representation to simplicial coefficient rings; its zeroth homotopy group
$\pi_0 \mathcal{R}$ recovers Mazur's deformation ring.
  We give evidence that these rings $\mathcal{R}$ occur in the wild: For
suitable Galois representations, the Langlands program predicts that $\pi_0
\mathcal{R}$ should act on the homology of an arithmetic group. We explain how
the Taylor--Wiles method can be used to upgrade such an action to a graded
action of $\pi_* \mathcal{R}$ on the homology.
