Smooth muscle cells (SMCs) play an important role in vascular development and disease. Vascular SMCs undergo profound changes in phenotype in response to environmental cues that aid in development and vascular repair, but perturbations in the process contribute to many cardiovascular diseases, including atherosclerosis, restenosis, and hypertension. Therefore, it is critical to determine the mechanisms that control their phenotype. We have previously shown that the small GTPase RhoA promotes SMC differentiation through an SRF-dependent mechanism, and the RhoA effectors mDia1 and mDia2 mediate this effect through actin polymerization-dependent localization of the myocardin-related transcription factors (MRTFs). The primary aim of this dissertation is to further dissect the contributions of mDia signaling to SMC phenotype in vivo and in the nuclear compartment of the cell. Using a Cre-based approach, we overexpressed a dominant-negative mDia variant specifically in SMCs in a mouse model. We determined that inhibition of mDia signaling impaired cardiac structure and normal vessel morphology in a subset of developing mice. Adult mice exhibited altered SMC marker gene expression and abnormal SMC migration, indicating that mDia signaling has roles in both SMC differentiation and migration. In vitro studies revealed that mDia variants that preferentially localized to the nucleus significantly enhanced SMC-specific transcription, while those that were less nuclear exhibited reduced SMC transcriptional activity. These observations were also recapitulated with other RhoA signaling genes, implicating a model whereby nuclear RhoA signaling promotes SMC differentiation. Finally, little is known about the mechanisms regulating the expression of the RhoA signaling gene program. Using DNase I hypersensitivity methods coupled to high-throughput sequecing, deletion and mutation analysis, and a transgenic LacZ approach, we present novel evidence that this signaling pathway, including mDia2, may be regulated in an SRF-dependent manner. Because RhoA signaling promotes SRF-dependent transcription, we hypothesize that this mechanism serves as an auto-regulatory feed-forward circuit to promote SMC differentiation.