The grading of oligodendrogliomas has proven to be a difficult undertaking complicated by several factors. Most important is the progressive nature of low grade oligodendrogliomas to anaplastic oligodendrogliomas, both biologically and histologically. In the absence of characteristic histotypic markers, the characterization of the oligodendroglial constituents themselves has proven to be subjective. This fact is best exemplified by the variable frequencies (from 5 to 25 percent of cases) of oligodendrogliomas noted by several groups investigating the occurrence of subtypes of gliomas (reviewed in Chapter 12). Another complicating factor seems to be the remark-able sensitivity of these tumors to therapies, a sensitivity that appears to be heightened among the so-called anaplastic types (Fortin et al., 1999; Bauman et al., 2000). However, to understand where the state of the art of oligodendroglial grading exists, it is important to review its history.