Many electronic structure methods can use density fitting (DF) to approximate integrals and thereby increase computational efficiency. These methods include, but are not limited to, Hartree-Fock (HF), density functional theory, complete active space self-consistent field, and second-order Møller-Plesset perturbation theory. Some authors refer to density fitting as resolution-of-the-identity (RI), but we shall use DF exclusively herein. In DF-HF, the products of orbital basis functions are approximated using auxiliary basis functions in both the Coulomb (J) and exchange (K) matrices. It has been demonstrated that auxiliary basis sets fit against the exchange matrix can also be used in the fitting of the Coulomb integrals, with the resulting auxiliary basis sets suffixed JKFit. Such auxiliary sets have been optimized for use with various orbital basis sets and are available for many elements of the Periodic Table. Automated methods for the generation of auxiliary basis sets have also been proposed, with some of these also being able to produce an auxiliary basis set on-the-fly as part of an electronic structure calculation. While such automated methods are both convenient and accurate, the auxiliary sets generated in this way tend to be larger than those optimized to specifically match a given orbital basis, increasing computational cost. DF-HF is often used to provide the reference wavefunction for local correlation methods, which aim to reduce the steep scaling of computational effort with respect to system size by exploiting the local nature of electron correlation. A detailed review of such methods is beyond the scope of this article, but a number of different approaches to using locality for reduced scaling are the subject of a recent book. Of particular interest here are explicitly correlated (F12) local coupled cluster methods using pair natural orbitals, such as the PNO-LCCSD(T)-F12 or DLPNO-CCSD(T) F12 methods. By including terms that depend explicitly on inter-electronic distance, modern F12 methods significantly reduce the basis set incompleteness error (BSIE) in the correlation energy, and reduce the domain error associated with local methods. This means that the density fitting error at the Hartree-Fock level, which is usually insignificant with canonical correlation treatments, can become an important source of error in the explicitly correlated local correlation case. The correlation consistent polarised valence n-zeta (cc-pVnZ) basis sets are often used in high-accuracy wavefunction-based calculations, partially due to their construction that systematically approaches the complete basis set (CBS) limit as n is increased. For the second-row elements Al-Ar, it was first noted by Bauschlicher and Partridge, and subsequently Martin, that unacceptably high errors in bind-ing energies could result from using the cc-pVnZ basis sets, even at the HF level. The inclusion of an additional "tight" (large exponent) d-exponent was later demonstrated to ameliorate this problem, and the resulting cc-pV(n+d)Z basis sets should be used for calculations involving the elements aluminium through argon. More recent correlation consistent basis sets for these elements, such as the cc-pVnZ-F12 family designed for explicitly correlated calculations, include the tight-d functions as a matter of course.