Several panelists stressed the need for care in defining observational criteria for considering a system interacting, beyond some kind of morphological peculiarity. Inevitably, some theoretical guidance is needed in this kind of interpretation. For example, narrow bridges and tails are well understood as results of certain kinds of interactions involving disk systems. Heckman stressed that such features should be seen in the old stellar population (the “Crimson Tide” approach) to avoid confusion with line emission or modest amounts of recent star formation that might have been influenced by hydrodynamical processes. Systems of these kinds will generally have an identifiable companion, perhaps tidally stripped or with its own tidal distortions, allowing further consistency checks on an interaction.