  We study barycenters of $N$ probability measures on $\mathbb{R}^d$ with
respect to the $p$-Wasserstein metric ($1<p<\infty$). We prove that
  -- $p$-Wasserstein barycenters of absolutely continuous measures are unique,
and again absolutely continuous
  -- $p$-Wasserstein barycenters admit a multi-marginal formulation
  -- the optimal multi-marginal plan is unique and of Monge form if the
marginals are absolutely continuous, and its support has an explicit
parametrization as a graph over any marginal space.
  This extends the Agueh--Carlier theory of Wasserstein barycenters [SIAM J.
Math. Anal. 43 (2011), no.2, 904--924] to exponents $p\neq 2$. A key ingredient
is a quantitative injectivity estimate for the (highly non-injective) map from
$N$-point configurations to their $p$-barycenter on the support of an optimal
multi-marginal plan. We also discuss the statistical meaning of $p$-Wasserstein
barycenters in one dimension.
