  Correlations in interacting many-particle systems can lead to the formation
of clusters, in particular bound states and resonances. Systematic quantum
statistical approaches allow to combine the nuclear statistical equilibrium
description (law of mass action) with mean-field concepts. A chemical picture,
which treats the clusters as distinct entities, serves as an intuitive concept
to treat the low-density limit. Within a generalized Beth-Uhlenbeck approach,
the quasiparticle virial expansion is extended to include arbitrary clusters,
where special attention must be paid to avoid inconsistencies such as double
counting. Correlations are suppressed with increasing density due to Pauli
blocking. The contribution of the continuum to the virial coefficients can be
reduced by considering clusters explicitly and introducing quasiparticle
energies. The cluster-virial expansion for nuclear matter joins known
benchmarks at low densities with those near saturation density.
