Harding's classification takes credits for defining the homogeneous phenotypes that have been essential for the genetic linkage studies and it is still useful for didactic purposes. The advances in pathogenetic knowledge make it now possible to modify Harding's classification. Five main pathogenetic mechanisms may be distinguished: 1) mitochondrial; 2) metabolic; 3) defective DNA repair; 4) abnormal protein folding and degradation; 5) channelopathies. The present attempt to classify ataxia disorders according to their pathogenetic mechanism is a work in progress, since the pathogenesis of several disorders is still unknown. A pathogenetic classification may be useful in clinical practice and when new therapeutic strategies become available.