Seedling characteristics between germination and the two leaf stage are principally responsible for variation in early leaf area development, or vigor, among temperate cereals. There is compelling evidence that this greater vigor confers substantial benefits in crops grown in dryland Mediterranean-type environments. Experiments were conducted to determine the important factors that are likely to be responsible for variation in early vigor among barley (Hordeum vulgare L.), bread wheat (Triticum aestivum L.), durum wheat (T. turgidum var durum), triticale (Triticosecale Wittm.), and oat (Avena saliva L.) cultivars. Variation among and within species in the following factors was examined: (i) time to emergence, (ii) efficiency of utilization of seed reserves, (iii) shoot biomass formed from seed reserves and attributed to a larger plumule relative to the radicle, (iv) embryo size, (v) size of leaf epidermal cells, and (vi) leaf expansion rate during the lower temperatures in winter. Seeds of a similar mass were used in all experiments to eliminate the effects of seed size. The superior vigor of barley and triticale compared with the two wheat species and oat previously found in field experiments were confirmed. The size of the embryo was the single most important factor to account for differences in vigor among the species. It accounted for 90% of the variation in vigor between the species. Earlier emergence in barley accounted for the remaining variation in vigor. The breadth of the first seedling leaf is suggested as an indirect estimate of early vigor that integrates embryo size and specific leaf area and could be used in a breeding program to increase the vigor of cereals.