Cosmological simulations unequivocally predict that cold accretion is the primary growth mechanism of galaxies. The accretion of cold gas occurs via cosmic filaments that transports metal-poor gas onto galaxies, providing their fuel to form stars. Observationally, it is quite clear that galaxy gas-consumption timescales are short compared to the age of the Universe, therefore galaxies must acquire gas from the surroundings to continue to form stars. However, observational data have yet to conclusively show that gas accretion flows are ubiquitously occurring in-and-around galaxies at any epoch. Since the first discoveries of circumgalactic gas around star-forming galaxies (Boksenberg & Sargent, 1978; Kunth & Bergeron, 1984; Bergeron, 1986), we have wondered where does this gas come from and if/how it drives galaxy growth and evolution.
This Chapter reviews the current observational evidence and signatures of cold accretion onto star-forming galaxies. In the following sections, we review the data that suggest accretion is occurring and cover the main topics of circumgalactic gas spatial distribution, kinematics and metallicity. In a few cases, there exists the combination of all of the above which provides the most tantalizing evidence of cold accretion to date. This Chapter primarily focuses on observations using background quasars or galaxies as probes of the circumgalactic medium around intervening foreground galaxies.