  We discuss a non-perturbative $T$-matrix approach to investigate the
microscopic structure of the quark-gluon plasma (QGP). Utilizing an effective
Hamiltonian which includes both light- and heavy-parton degrees of freedoms.
The basic two-body interaction includes color-Coulomb and confining
contributions in all available color channels, and is constrained by
lattice-QCD data for the heavy-quark free energy. The in-medium $T$-matrices
and parton spectral functions are computed selfconsistently with full account
of off-shell properties encoded in large scattering widths. We apply the
$T$-matrices to calculate the equation of state (EoS) for the QGP, including a
ladder resummation of the Luttinger-Ward functional using a matrix-log
technique to account for the dynamical formation of bound states. It turns out
that the latter become the dominant degrees of freedom in the EoS at low QGP
temperatures indicating a transition from parton to hadron degrees of freedom.
The calculated spectral properties of one- and two-body states confirm this
picture, where large parton scattering rates dissolve the parton quasiparticle
structures while broad resonances start to form as the pseudocritical
temperature is approached from above. Further calculations of transport
coefficients reveal a small viscosity and heavy-quark diffusion coefficient.
