  We have observed Fermi polarons, dressed spin down impurities in a spin up
Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in
the impurities' rf spectrum that emerges from a broad incoherent background. We
determine the polaron energy and the quasiparticle residue for various
interaction strengths around a Feshbach resonance. At a critical interaction,
we observe the transition from polaronic to molecular binding. Here, the
imbalanced Fermi liquid undergoes a phase transition into a Bose liquid
coexisting with a Fermi sea.
