Metal enrichment of the high-z intergalactic medium

Recent observations show the presence of metals in low-density Ly$\alpha$ forest absorbers at high redshift ($z\sim 3$). It remains still far from being clearly understood what mechanisms spread metals over Mpc scales from the parent galaxies, whether metals are homogeneously distributed in the intergalactic medium (IGM), how metallicity of the IGM does evolve. These questions are briefly addressed in this review.