It has been broadly accepted that barley performs better than wheat, particularly under severe stress conditions of the Mediterranean basin. However, this assumption is not based on rigorous evidence. This study compared grain yield, grain yield components and biomass accumulation performances of barley, bread wheat and durum wheat under a wide range of water and nitrogen (N) availabilities on a location in the Mediterranean basin during four consecutive growing seasons (2003/04, 2004/05, 2005/06 and 2006/07). The different experimental treatments enabled the comparison of the three species under wide grain yield ranges. Yields under these treatments ranged from 0.9 to 10.2 Mg ha−1. Grain number per unit area better explained grain yield differences in the three species across experimental years and treatments. Photothermal quotient (PTQ) during the critical growth period (from 20 days pre- to 10 days post-flowering) explained the differences in grain number for all three species (r2 = 0.64; r2 = 0.84 and r2 = 0.56 for barley, bread wheat and durum wheat, respectively). Biomass differences between wheat and barley were generated mainly by their differences in post-flowering growth. Post-flowering growth was positively and directly related to the number of grains set per m2. It was confirmed that it is not universally true that barley yields better than wheat under Mediterranean environments.