Intensity factors in molecular spectra

The interest in precise knowledge of the intensity factors for investigating molecular structure is pointed out and illustrated by some typical recent examples. Diatomic intensity factors are derived. Starting from pure coupling-case wave functions, the intensity factors related to transitions between states belonging to $a_{\alpha }$, $a_{\beta }$, $b_{\beta J}$, $b_{\beta S}$, and $b_{\beta N}$ coupling cases, in the spin-forbidden or spin-allowed hypothesis, are derived. Cases are considered for which hyperfine structure is fully observed, partially observed, or absent.