  We study resonant tunneling through quantum-dot systems in the presence of
strong Coulomb repulsion and coupling to the metallic leads. Motivated by
recent experiments we concentrate on (i) a single dot with two energy levels
and (ii) a double dot with one level in each dot. Each level is twofold
spin-degenerate. Depending on the level spacing these systems are physical
realizations of different Kondo-type models. Using a real-time diagrammatic
formulation we evaluate the spectral density and the non-linear conductance.
The latter shows a novel triple-peak resonant structure.
