After a brief historical review of the discovery of helium in the terrestrial atmosphere, the production mechanisms of the isotopes He4 and He3 are discussed. Although the radioactive production of He4 in the Earth is well understood, some uncertainty still exists for the degassing process leading to an atmospheric influx of (2.5 ±1.5) × 106 atoms cm-2 s-1. Different production mechanisms are possible for He3 leading to an influx of (7.5±2.5) atoms cm-2 s-1. Observations of helium in the thermosphere show a great variability of this constituent. The different mechanisms proposed to explain the presence of the winter helium bulge are discussed. Since helium ions are present in the topside ionosphere and in the magnetosphere, ionization mechanisms are analyzed. Owing to possible variations and uncertainties in the solar UV flux, the photoionization coefficient is (8±4) × 10-8 s-1. Finally, the helium balance between production in the earth and loss into the interplanetary space is discussed with respect to the different processes which can play an effective role.