Resonantly interacting Fermi system has attracted growing interests since its

realization in ultracold atoms using the Feshbach resonance. One spectacular few-body

phenomenon appearing at resonance is the Efimov states, an infinitenumber of 3-body

bound states with discrete scale invariance. I first review 3-body physics in a twocomponent

Fermi system, and show when 3-body bound states, such as the Efimov

states, can appear. I then show how such few-body knowledge can give new insights

into many-body physics in a two-component Fermi system. I first show that there exists

a stable many-body ground state composed of trimers. This trimer many-body phase is

found to be a three-component Fermi liquid with SU(3) symmety. It is robust against

recombination losses and can be realized in future cold atom experiments. Finally, I

talk about our recent study on the virial expansion in the unitary Fermi gas.