The mass-concentration relation in massive galaxy clusters at redshift
  ~1
  Mass and concentration of clusters of galaxies are related and evolving with
redshift. We study the properties of a sample of 31 massive galaxy clusters at
high redshift, 0.8 < z < 1.5, using weak and strong lensing observations.
Concentration is a steep function of mass, c_{200} ~M_{200}^{-0.83 +-0.39},
with higher-redshift clusters being less concentrated. Mass and concentration
from the stacked analysis, M_{200}=(4.1+-0.4)x10^{14}M_Sun/h and
c_{200}=2.3+-0.2, are in line with theoretical results extrapolated from the
local universe. Clusters with signs of dynamical activity preferentially
feature high concentrations. We discuss the possibility that the whole sample
is a mix of two different kinds of haloes. Over-concentrated clusters might be
accreting haloes out of equilibrium in a transient phase of compression,
whereas less concentrated ones might be more relaxed.
