The olive tree is the cultivated form of the wild oleaster, both of which belong to the subspecies europaea of Olea europaea and are naturally distributed all around the Mediterranean Sea. In addition to these, some trees escaped from cultivation resemble oleasters by their physiognomy. No specific morphologic marker unambiguously differentiates the three forms. Olive cultivars today show a wide diversity in their morphology and phenology. Olives are important economically in Mediterranean countries, and olive oils carrying the taste and aroma of the fruit are the base of a new gastronomy also economically significant. Olive trees fashion landscapes and prevent erosion, and a social culture is associated with their products. Oleasters are endangered due to recurrent gene flow from the olive tree, human impact on forests and climate change. Olive cultivars result from a long process of selection in diverse environments that have had different cultural practices and traditions ever since the olive tree began accompanying human migration in the Neolithic period. The first domestication had occurred by -5800 B.P. around the eastern Mediterranean basin. Olive cultivars are deeply differentiated according to ultimate use — for oil, table or mixed. Their origins are unknown and the country of origin is only an indication of where they come from. Cultivars and oleasters are wind-pollinated and outcrossing is the rule. Molecular markers have recently made it possible to study the diversity of olive trees and thus to attempt to verify myths and beliefs about their origins. Relations between cultivars can now be established with several types of markers. Domestication events must have appeared in several sites around eastern and western Mediterranean localities since cultivars have inherited cytotypes of local oleasters, and this probably occurred simultaneously. Human migrations displaced cultivars, leading to gene flow: local oleasters generate new forms and new cultivated genotypes. Cytoplasmic markers show at least four separate origins of olive trees from oleasters, and SSRs show at least seven. Molecular markers have enabled us to show that each cultivar corresponds to one clone with a few exceptions. This means that cultivars were propagated from a single tree, with some exceptions that may be due to mixing two or three sister progenies of one tree. Although oleasters originate from seven different refuge areas, gene flow caused by cultivar displacement has disturbed this structure. Some cultivars also have their origin in some of these primary populations but others appeared as hybrids between two or three of these zones. This suggests that gene flow occurred between local oleasters and cultivars introduced by human migrations