Now that airbags are the accepted solution for protecting occupants in frontal impacts, and now that safety sells cars, it is natural to look closely at the second largest killer of automobile occupants, side impacts. This paper develops the theory of sensing side impacts based on the assumption that airbags will soon be used for side impact protection. The trade-offs between the various sensor technologies are discussed including electronic and mechanical sensors. For mechanical sensors, fluid damped, undamped and crush sensing switches are compared. Finally, the requirements for a successful predictive sensor will be presented.