One way to improve a program's efficiency is to ensure that the data are accessed as rapidly as possible in the innermost loops of time-consuming routines. This is particularly true for modern computer architectures which rely on pipelining to boost performance---complicated memory addressing can clog up the pipe. These notes are about various schemes for numbering (or indexing) orbitals; as you might expect, different cases call for different strategies. We will also discuss the indexing of various quantities which are in turn indexed by orbitals (in particular, the one- and two-electron integrals). Since these notes are written primarily for the benefit of members of the Schaefer group, we will make some specific references to the PSI ab initio package.