Metallicity and ionization condition of ISM in star-forming galaxies at high redshift

I present recent progresses on the mass-metallicity relation and ISM conditions at z>2-3, with an emphasis on the mass-metallicity relation and its relation on SFR at z~3.3 based on our near-IR spectroscopic observations with Keck/MOSFIRE. I also present future prospects in near future on the focus of basic measurements and possible advancement with future instruments and telescopes