  Nuclei in dense matter are influenced by the medium. In the cluster mean
field approximation, an effective Schr\"odinger equation for the $A$-particle
cluster is obtained accounting for the effects of the surrounding medium, such
as self-energy and Pauli blocking. Similar to the single-baryon states (free
neutrons and protons), the light elements ($2 \le A \le 4$, internal quantum
state $\nu$) are treated as quasiparticles with energies $E_{A,\nu}(P; T,
n_n,n_p)$ that depend on the center of mass momentum $\vec P$, the temperature
$T$, and the total densities $n_n,n_p$ of neutrons and protons, respectively.
We consider the composition and thermodynamic properties of nuclear matter at
low densities. At low temperatures, quartetting is expected to occur.
Consequences for different physical properties of nuclear matter and finite
nuclei are discussed.
