Varanoidea is a monophyletic group of anguimorph lizards, comprising the New World helodermatids, the Bornean earless monitor Lanthanotus borneensis, and the Old World monitors (Varanus). I use mitochondrial DNA sequences and extensive taxonomic sampling to test alternative hypotheses of varanoid relationships. The most parsimonious hypothesis confirms the monophyly of Varanoidea (Heloderma, Lanthanotus, and Varanus) and Varanus, as well as the sister-taxon relationship of Varanus and Lanthanotus. The relationships among Varanus species differ in several respects from previous hypotheses. Three major lineages are recognized within Varanus: an African clade basal to the rest of the group, an Indo-Asian clade, and an Indo-Australian clade. Within the last lineage, the endemic Australian dwarf monitors (Odatria) form a clade sister to the large Australian monitors (the gouldii group). Tests of the effects of rate heterogeneity and homoplasy demonstrate that putative process partitions of data are largely congruent with one another and contribute positive support to the overall hypothesis.