An account of the first theoretical studies on the role of back-bonding in carbocation chemistry is presented. The bonding structure of EH₃⁺ and EH₅⁺ (E=C, Si, Ge) species may be rationalised by means of a back-bonding interaction between EH⁺ and EH₃⁺ moieties and H₂. Back-bonding interaction between C₆H₅⁺ and H₂ is also crucial for the formation of the benzenium ion, C₆H₇⁺. Back-donation determines the H₂ elimination channels from benzenium ion and ethyl cation as well. In effect, the hydrogen molecule can be eliminated only when the two C-H bonds involved stretch and the back-bonding is weak. In the case of ethyl cation this condition implies that the simultaneous stretching of the two C-H bonds takes place out of phase with the  C-C elongation. Thus, the concept of back-donation, which plays a fundamental role in explaining the bonding between certain ligands and transition metal atoms, between certain nonmetals, and the bonding structure of π and σ complexes is also essential for understanding carbocation chemistry.