We have discussed earlier that, within an atom, there are magnetic moments owing to the orbital and spin motion of the electrons. Magnetic interaction between these moments was discussed as spin- orbit interaction that caused each term of a given \(l\) split into terms each with same \(l\) but different \(j(l \pm s\) value. In other words, there is a net magnetic moment, \(\mu_j\) in an atom. When atom is subjected to external magnetic field, \(H\) of few thousand gauss (1 Tesla = \(10^{4}\) Gauss), there will be magnetic interaction between \(H\) and \(\mu_j\) resulting in further splitting of the spectral lines. Such splitting, in the presence of magnetic field of few thousand gauss, of the sodium \(D\) lines was first observed by Pieter Zeeman, a Dutch physicist, in 1896; the effect was subsequently named after him – Zeeman Effect. The Beauty of these findings is that splitting was observed not only before Bohr’s theory of hydrogen atom (1913) but was also before the discovery of electron in 1897. The importance of the Zeeman Effect can be judged by its application to the fields of NMR, ESR, MRI, Mӧssbauer Spectroscopy etc., each is now a well established field of study in itself.