Vibrational Stark effects, which are the effects of electric fields on vibrational spectra, were measured previously for the C-N stretch mode of several small nitriles, yielding difference dipole moments, difference polarizabilities, and transition polarizabilities for each species [Andrews, S. S.; Boxer, S. G. J. Phys. Chem. A 2000, 104, 11 853]. This paper explains the physical origins of the observed Stark effects using two theoretical models and, in the process, computes several molecular parameters for each nitrile. A model with a single vibrational mode, developed with perturbation theory, is found to explain most of the experimental Stark effects. Because it cannot account for coupling between modes, which is ubiquitous and important for resonant vibrations and for combination mode absorption, another model is developed which considers multiple vibrational modes and three spatial degrees of freedom. It is found that difference dipole moments arise from a combination of mechanical anharmonicity and electronic perturbations of chemical bonds, where the two factors have about equal magnitudes for nitriles. Transition polarizabilities are dominated by the effects of electronic polarizability of the sample molecule, which alters the partial charges on atoms in an electric field. Stark effects of resonant transitions are found to be equal to linear combinations of the effects for the basis states, which explains an experimental observation. For overtone and combination transitions, Stark shifts are predicted to be the sums of the shifts of the component transitions. Absolute overtone intensities can be calculated from difference dipole results, which is experimentally verified. In summary, these theories largely explain the physical origins of observed vibrational Stark effects and can predict Stark effects for a wide variety of other systems.