  The physics of the intracluster medium, in particular the values for the
thermal conductivity and the viscosity are largely unknown and subject to an
ongoing debate. Here, we study the effect of viscosity on the thermal state of
the intracluster medium using three-dimensional cosmological simulations of
structure formation. It is shown that viscosity, provided it is not too far off
from the unmagnetised Spitzer value, has a significant effect on cluster
profiles. In particular, it aids in heating the cool cores of clusters. The
central cooling time of the most massive clusters in our simulation is
increased by more than an order of magnitude. In large clusters, viscous
heating may help to establish an entropy floor and to prevent a cooling
catastrophe.
