A key aspect of plant growth is the synthesis and deposition of cell walls. In specific tissues and cell types including xylem and fiber, a thick secondary wall composed of cellulose, hemicellulose, and lignin is deposited. Secondary cell walls provide a physical barrier that protects plants from pathogens, promotes tolerance to abiotic stresses, and fortifies cells to withstand the forces associated with water transport and the physical weight of plant structures. Grasses have numerous cell wall features that are distinct from eudicots and other plants. Study of the model species Brachypodium distachyon has helped us begin to understand the internal and external cues that regulate the synthesis of grass secondary cell walls. In this dissertation, I investigate the function of two transcription factors in regulating cell wall biosynthesis, SWIZ and KNOB7. SWIZ controls wall synthesis and plant growth in response to external mechanical force. In response to touch, SWIZ protein moves into the nucleus, a translocation that is modulated by the level of bioactive gibberellic acid in the cell. Positive and negative perturbation of SWIZ results in shorter plants with thicker fiber cell walls, phenotypes that are enhanced in plants treated with regular mechanical stimulus during growth. KNOB7 is orthologous to the characterized cell wall regulator AtKNAT7 in Arabidopsis thaliana. KNOB7 negatively regulates fiber wall thickness and lignification, as is observed in AtKNAT7, but KNOB7 shows unique control of lignin composition, hydroxycinnamic acid content, and cell wall polysaccharide content. These observations may reflect control of grass specific cell wall characteristics not present in eudicots, such as high levels of wall bound hydroxycinnamic acids and the prevalence of heteroxylan polysaccharides. Together, these insights from SWIZ and KNOB7 function further our understanding of how grasses regulate their growth and secondary cell wall synthesis.