Tumors of the same class should share a unique combination of features, which distinguishes them from all other classes. Classification, therefore, is the process of defining different tumor classes and correlating these with each other in a logical manner based on the putative cell of origin. Classification also involves “grading” of tumor cell morphological features and consideration of their immunohistochemical and genetic profiles. It does not include tumor “staging,” which determines the extent of tumor involvement and hence the type of treatment required (i.e., local versus systemic treatment).