Significance Plants both control and are controlled by the global climate. Grasses in natural and agricultural systems participate in the exchange of atmospheric CO ₂ for biosphere-derived oxygen and water vapor via microscopic epidermal valves (stomata), but how these stomata are made in grasses is unknown. Using genetic screens and targeted genome editing, we identify and characterize master transcriptional regulators of stomatal initiation in the wheat relative Brachypodium . Surprisingly, the unique stomatal form and pattern of grasses is regulated by orthologs of Arabidopsis stomatal basic helix–loop–helix (bHLH) transcription factors, although the function of individual genes and regulation of their protein products have diverged. This finding suggests that the stomatal core bHLH transcription factors are excellent breeding targets to enhance performance in grasses.