In this paper a new interaction measure for stable multivariable systems is introduced. The interaction measure, the Hankel Interaction Index, is a dynamic extension of the underlying idea in the classic Relative Gain Array (RGA). The new index is based upon the Hankel norm of the SISO elementary subsystems built from the original MIMO system. The main advantage of the Hankel Interaction Index is its ability to quantify frequency dependent interactions and that it can be used for input-output pairing. Several examples are included to illustrate the new ideas presented.