We discuss the concept of cation affinities (CA) and provide an overview of topical trends throughout the periodic table, inferred from state-of-the-art relativistic quantum-chemical computations. The CA of a base B
for a cation Y
is the energy or enthalpy required to dissociate the complex BY
into molecular fragments B
. Probably the best-known CA is the proton affinity (PA) of Lewis bases. We extend this concept here to include methyl cation (MCA) and other alkyl cation affinities (ACA) as well as alkali metal cation affinities (AMCA). The Lewis bases covered herein are the anionic and neutral element hydrides B
and B = XH
, respectively, as well as methyl-substituted variants thereof. The element “X” in our model Lewis bases covers the maingroup elements of groups 14–18 in rows 1–6 of the periodic table. Emerging trends are analyzed and explained in terms of quantitative molecular orbital (MO) theory as contained in Kohn–Sham density functional theory (KS-DFT). Making the often implicitly used idea of a CA explicit serves a more rational design of compounds with a particular affinity for, or reactivity toward, other species throughout the molecular sciences, from inorganic via organic to biological chemistry.