A great deal of contention can be found within the published literature on grounding and the symbol grounding problem, much of it motivated by appeals to intuition and unfalsifiable claims. We seek to define a formal framework of representa tion grounding that is independent of any particular opinion, but that promotes classification and comparison. To this end, we identify a set of fundamental concepts and then formalize a hierarchy of six representational system classes that correspond to different perspectives on the representational requirements for intelligence, describing a spectrum of systems built on representations that range from symbolic through iconic to distributed and unconstrained. This framework offers utility not only in enriching our understanding of symbol grounding and the literature, but also in exposing crucial assumptions to be explored by the research community.