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Seven thousand years of barley cultivation under the environmental hardships typical of the Mediterranean climate have generated genetic singularity of the Spanish barleys, consistently reported in the literature. From the Spanish National Collection of 2289 accessions, a core subset with 159 landraces and 16 old varieties was constituted. Twenty-seven characters were evaluated for the core collection, to define the structure of the diversity. Several evaluation trials were carried out in 1999—2000, whereas yield trials were performed in earlier years. Phenotypic diversity was large for most of the characters studied. Comparisons of genetic diversity between the core and the original collections suggested that the core is a good representation of the existing diversity in the BNG. Comparisons with results of studies on Spanish materials from other collections seem to indicate that the Spanish diversity is not well represented in some world collections. Principal component analyses for quantitative and qualitative characters revealed a clear distinction between two- and six-row cultivars, and also between landraces and commercial varieties. Geographical origins of the landraces were correlated with grain yield, heading date, duration of grain filling period, and growth class. In relation to diseases, altitude played an important role on the resistance to powdery mildew and brown rust. For brown rust, all the resistant landraces came from low altitudes. These geographical gradients seemed consistent with prior knowledge about barley adaptation, and would confirm the agreement between passport data and true adaptive origin of these landraces from a geographical point of view.