1 Overview of dispersion in DFT Sanibel Symposia at the dawn of the 21st century were, in the hazy memory of one author, characterized by an obsession with potential energy curves for rare-gas dimers, computed using density functional theory (DFT). Already by 1995 there was evidence of problems for dispersion-bound systems, evidencing a growing recognition of semilocal DFT's failure to describe long-range electron correlation and with it, dispersion interactions. This led to early efforts to describe dispersion using coupled Kohn-Sham response theory, and to incorporate those ideas into an exchangecorrelation (XC) functional that would be applicable to van der Waals (vdW) complexes. 9-15 (This history has been reviewed elsewhere. ) These efforts would eventually bifurcate, with one coterie continuing the quest for a universal density functional to describe dispersion while others pursued near-term practical solutions based on ad hoc dispersion corrections to existing XC functionals. The former approach is exemplified by the so-called "van der Waals density functional" (vdW-DF) of Langreth and Lundqvist, along with the simplified VV10 functional of Vydrov and Van Voorhis. Each of these is a nonlocal correlation functional that contains the correct ingredients to describe long-range dispersion between non-overlapping charge densities. VV10 has since been incorporated into a new generation of XC functionals that perform remarkably well for nonbonded interactions. An alternative to finding a universal density functional is to build a model for dispersion that can be incorporated into (or added onto) an existing XC functional. Among these, Grimme's dispersion corrections have become ubiquitous. In practice, these amount to atom-atom potentials of the form -C 6 /R 6 and -C 8 /R 8 , along with three-body (triatom) corrections, added to a semilocal or hybrid functional in the spirit of much earlier "Hartree-Fock plus dispersion" methods. Short-range damping functions forestall any catastrophes as R → 0 and also mitigate potential double-counting of electron correlation in regions of space where the monomer densities still overlap. In this dispersion-corrected "DFT+D" approach, the double-counting issue has important ramifications for how the correction (+D) should be interpreted. This is a main point of discussion in the present chapter.