The Casimir effect is commonly defined as the change in the interaction energy or force between two plates in the absence of external fields.

For microscopic systems, like atoms and molecules, a similar effect is seen, where the Casimir-Polder potential appears in the case, for example, of two ground-state systems that are well-separated with respect to the wavelength of a photon characterizing a system's energy.

I will discuss the theory and calculation of the Casimir-Polder potential for several cases of interest in atomic, molecular, and optical physics.