The goals of this document are to provide an introduction to the theory behind the calculations of infrared intensities using Psi. Section two derives an experimental formula for intensity as well as a derivation of the preferred unit, km/mol. Section three will be defining a new coordinate system based on placing the center of mass at the origin and having the system rotate with the molecule. Herein, the infamous Eckart-Sayvetz conditions will be derived. Section four derives a formula for infrared intensities as a function of the dipole moment derivatives. The effects of isotopic substitution will also be described here as well. Section five will have a description of how Psi incorporates these conditions into finite displacement calculations. Finally, section six details a protocol for carrying out finite displacement calculations using Psi to determine infrared frequencies and intensities at correlated theories. It is designed with the beginning Psi user in mind.