The outer parts of spiral and dwarf irregular galaxies usually have a regular structure with an exponentially declining surface brightness in FUV, optical, and near-infrared passbands, somewhat flatter or irregular radial profiles in atomic gas, and frequent evidence for azimuthal asymmetries. Models suggest that these outer parts form by a combination of gas accretion from the halo or beyond, in situ star formation, and stellar scattering from the inner disk. The exponential shape is not well understood, but cosmological simulations get close to exponential shapes by approximate angular momentum conservation. Wet mergers also get exponentials after the combined stellar systems relax, and stellar scattering from gas irregularities and spiral arm corotation resonances get exponentials too, all probably for different reasons.
Star formation persists in far outer disks without any qualitative change in the properties of individual star-forming regions. This happens even though the gas density is very low, Hα is often too weak to see, and the dynamical time is long. Gas also tends to dominate stars by mass in the outer parts, but the gas appears to be mostly atomic, making star formation difficult to understand in comparison to inner disks, where it is confined to molecular clouds.
Colour and age gradients suggest that most spiral galaxies have their earliest star formation in the inner disk, with a scale length that increases in time and an outward progress of gas depletion or quenching too. The result is a tendency for spirals to get bluer with increasing radius. Eventually the blue trend stops and spirals get redder after that. This gradient change occurs in all types of spiral galaxies, regardless of the exponential shapes of their radial profiles, and suggests a different process for the formation of inner and outer stellar disks. Most likely stellar scattering from the inner disk to the outer disk is part of the explanation, including stellar scattering from bars and spirals, but there could be other processes at work too, including minor mergers and interactions with other galaxies. Colour gradients in dIrrs are usually the opposite of those in spiral galaxies. Dwarfs tend to get systematically redder with radius in what looks like outside-in star formation. This could reflect an enhanced role for stellar scattering with the first star formation still near the centre, as for spirals, or it could result from radial gas accretion or other truly outside-in processes.
The advent of new surveys that probe galaxies to very faint stellar surface brightnesses, combined with new maps of atomic and molecular emission from the far-outer regions of galaxies, should help us to better understand the origin and evolution of galaxy disks.
Acknowledgements DAH appreciates the support of the Lowell Observatory Research Fund in writing this chapter.