The evolution of high-redshift galaxies is regulated by the balance between the inflow of fresh fuel that is required to form new stars and the outflow of metal-polluted material that is ejected from star-forming regions. For this reason, the circumgalactic medium between galaxy disks and the intergalactic medium has been recognized as one of the most fundamental components in a galaxy. In this talk, I will discuss recent progress at the interface between theory and observations to characterize the gaseous environment of distant galaxies. I will focus on the emergent theoretical picture according to which galaxies at high redshifts are fed by extended streams of cold gas in a smooth component and in merging satellites. I will compare and contrast the findings of numerical simulations with observations of high-redshift galaxies and I will discuss the prospects of mapping the circumgalactic medium with absorption line systems, presenting results from recent and ongoing observations.