Beautiful stellar disks only come to be with the accretion of gaseous clouds of star formation fuel. There are several reasons why this fuel is thought to be gradually accreted from a galaxy's halo. The first is that halo gas exists around our Galaxy and other spiral galaxies (e.g., Oort 1970; Thilker et al. 2004; Oosterloo, Fraternali & Sancisi 2007). Halo gas therefore appears to be a relatively common phenomenon and the gas velocities indicate it will not escape from the galaxy, but rather eventually fall towards the disk. The second reason is the metallicity distribution of the long-lived stars in the Galactic disk, which indicates low metallicity fuel must be continually accreted, i.e., the G-dwarf problem. The metallicity distribution of the G and K dwarfs in the solar neighborhood cannot be reproduced with simple closed box models, and the need for gaseous inflow for the majority of the life of the disk has persisted with further observations and increasingly complex chemical evolution models (e.g., Larson 1972; Fenner & Gibson 2003; Kotoneva et al. 2002; Chiappini, Matteucci & Romano 2001; Magrini, Corbelli & Galli 2007; Worthey et al. 2005). A third reason to bring in star formation fuel from the halo is the lack of radial gaseous inflow observed in the disk of spiral galaxies (Wong et al. 2004). Bringing the outer, relatively unenriched gas to the inner regions of the disk would potentially provide a fresh source of star formation fuel.
There are two main potential sources of gaseous fuel in a galaxy halo: gas-rich satellites and condensed material from the hot diffuse halo. The latter may be a combination of material left from the initial collapse of baryons into the dark matter halo, stripped material from the satellites that was integrated into this hot halo, and some level of galactic fountain material at low latitudes. In this review, we discuss these two reservoirs of disk star formation fuel with a focus on the Milky Way, given this is where we have the most information on the satellite distribution and the low column density halo gas. We will also discuss the halo gas of M31 and M33 in the context of its origin and fate.