Background: Quercus is recognized as a taxonomically complex genus, but also as a model clade in many important fields in biology, such that a good recognition of its species is necessary. The chosen species concept to use in Quercus will determine the empirical criteria used to recognize them, which will impact several areas of knowledge. Questions: What are the main sources of variation that hinder the delimitation of species in Quercus ? What species concepts we use explicitly to recognize species in Quercus ? What is the advantage of using different empirical criteria both integrally and simultaneously in delimitation of species of oaks? Studied species: Species of Quercus Method: Bibliographic review of the main sources of variation in Quercus , and the species concepts, specifically those used in Quercus . Results: Plasticity, convergence, hybridization and introgression, and incomplete divergence were identified as the main sources of variation in oaks. Taxonomic and ecological species concepts are those mainly and traditionally used in Quercus . Syngameons are important to know and understand the biology and evolution of Quercus species. These systems indicate that there are preserved genes that provide coherence and morphologic, ecologic and genetic identity to species, even if hybridization, backcrossing and introgression occur. Conclusions: Preserved genes that provide coherence to species, suggest using taxonomic, ecologic and genetic concepts to delimit problematic species in species complexes in Quercus . The simultaneous use of data that these concepts support (multicriteria analysis), will give more confidence to get closer to the nature of the species and build an integrative taxonomy.