Helium is the second most abundant element in the Universe, and, when singly ionized, is hydrogenic. This means HeII has a hydrogen-like absorption spectrum but with transition energies a factor of 4 higher. This places HeII Ly α forest lines deep into the ultraviolet, the consequences of which highly limit the redshift visibility of HeII studies – only favorable quasar sightlines can be used to study HeII Ly α and Ly β absorption. The column density ratio of HeII to HI is highly sensitive to the shape and intensity of the cosmic ultraviolet background (UVB), and thus is a key quantity for constraining the evolution and patchiness of the UVB. An Epoch of HeII Reionization stretching into the Cosmic Noon era provides insights into the appearance of the first quasars in the Universe. In this chapter, we describe the redshift visibility of HeII absorbers, discuss the cosmic impact of HeII absorption, and describe key observational results, including the so-called hardness parameter, the HeII Gunn-Peterson trough, and HeII Ly α spikes.