Presently, there exist two classical analytical results that provide information regarding the lateral stability of car/trailer systems. These results are the directional stability equation for the automobile and the frequency and damping relationships for yaw oscillations of a trailer towed by a vehicle of infinite mass. Neither of these classical results provides an adequate description of car/trailer dynamic behavior because the associated classical models do not allow for any car/trailer interaction.In this paper, basic analytical results for car/trailer lateral stability are developed. These results are validated by comparing critical speeds predicted by the new analytical solution with those obtained numerically using a standard eigenvalue technique. General observations based upon the analytical results are then presented.