  We present and test TesseRACt, a non-parametric technique for recovering the
concentration of simulated dark matter halos using Voronoi tessellation.
TesseRACt is tested on idealized N-body halos that are axisymmetric, triaxial,
and contain substructure and compared to traditional least-squares fitting as
well as two non-parametric techniques that assume spherical symmetry. TesseRACt
recovers halo concentrations within 0.3% of the true value regardless of
whether the halo is spherical, axisymmetric, or triaxial. Traditional fitting
and non-parametric techniques that assume spherical symmetry can return
concentrations that are systematically off by as much as 10% from the true
value for non-spherical halos. TesseRACt also performs significantly better
when there is substructure present outside $0.5R_{200}$. Given that
cosmological halos are rarely spherical and often contain substructure, we
discuss implications for studies of halo concentration in cosmological N-body
simulations including how choice of technique for measuring concentration might
bias scaling relations.
