  This paper presents a measurement of the underlying event activity in
proton-proton collisions at a center-of-mass energy of 13 TeV, performed using
inclusive Z boson production events collected with the CMS experiment at the
LHC. The analyzed data correspond to an integrated luminosity of 2.1 fb$^{-1}$.
The underlying event activity is quantified in terms of the charged particle
multiplicity, as well as of the scalar sum of the charged particles' transverse
momenta in different topological regions defined with respect to the Z boson
direction. The distributions are unfolded to the stable particle level and
compared with predictions from various Monte Carlo event generators, as well as
with similar CDF and CMS measurements at center-of-mass energies of 1.96 and 7
TeV respectively.
