Accurate species delimitation has critical implications for ecological and conservation studies; and for understanding factors driving diversification. However, a growing body of evidence indicates that morphology-based species circumspection in lichenized ascomycetes often fails to accurately represent the number of fungal species. The use of molecular data in lichen systematics provides an important alternative to traditional morphological characters for identifying natural groups and assessing evolutionary histories in challenging lichen taxa. In this work, I examined two common lichen-forming genera in western North America, Rhizoplaca and Xanthoparmelia, as models for investigating character evolution, species delimitation in morphologically and chemically diverse species, and identification of lineages in the early stages of divergence. Phylogenetic hypotheses were reconstructed to assess character evolution using sequence data from four nuclear ribosomal markers and fragments from two nuclear loci. I applied a multifaceted approach to delimit species in Rhizoplaca and Xanthoparmelia by assembling multiple lines of evidence using DNA sequence data, and genealogical and population genetic analyses. I have found that traditionally circumscribed species are not supported by molecular data. For example, in Rhizoplaca previously unrecognized lineages were identified within what has thus far been considered a single species. In contrast, morphologically and chemically distinct species within Xanthoparmelia were not supported by molecular data. Distinct medullary chemistries, growth forms, and the production of vegetative diaspores appear to have evolved independently multiple times in Xanthoparmelia. This work clearly indicates that morphological and chemical characters do not always accurately reflect lichen species diversity within even the best known and studied genera. My study of the Rhizoplaca melanophthalma species complex demonstrates that the genus Rhizoplaca, as presently circumscribed, is more diverse in western North American than previously thought. I present these analyses as a working example of species delimitation in morphologically cryptic lichenized fungi. In Xanthoparmelia diagnostic morphological and chemical characters have evolved in a highly homoplasious manner. In contrast to other studies documenting previously undiscovered fungal lineages masked within lichen species circumscribed by traditional morphological and chemical characters, my work suggests that species diversity has been overestimated in the lichen genus Xanthoparmelia.