The ongoing revolution in information technology drives advances in the ability to synthesize, manipulate, and probe nanometer-size materials. Among the new developments, single-quantum-level tunneling spectroscopy emerges as a powerful tool to study the electronic structure of individual metal nanograins. I will present a microscopic theory of electron tunneling in ultra-small aluminum grains, which helps understanding tunneling spectroscopy experiments. The observed high density of resonances and the asymmetry of the tunneling spectra agains voltage-bias reversal can be understood within the new theory. I will also show that the local electrostatic environment of metal grains significantly change the spectroscopy in the nanoscopic regime.