Liver fibrosis is a major cause of morbidity and mortality around the world. In the UK incidence of liver fibrosis is rapidly increasing. Diagnostic, prognostic and treatment strategies are relatively limited. The only treatment for end stage fibrosis is liver transplantation. Although our understanding of the pathogenesis of fibrosis is increasing, there are still no approved anti-fibrotic therapies. Identifying key pathways may lead to the development of anti-fibrotics. These could slow or reverse the disease. Fibrosis of the liver involves a complex cascade of cellular and molecular responses, including an inflammatory response, mechanical forces and the activation of hepatic stellate cells (HSCs). HSCs are the main cell type responsible for the production and progressive accumulation of extracellular matrix (ECM) proteins, the central characteristic of liver fibrosis. Several pathways have been shown to cause the activation of HSCs. In this thesis three transcriptional regulators, sex-determining region Y-box 9 (SOX9), yes-associated protein 1 (YAP1) and circadian locomotor output cycles kaput (CLOCK) are identified as important drivers of HSC activation and liver fibrosis progression.The transcription factor SOX9 plays an important role in the development of several organs and regulates extracellular matrix (ECM) proteins during chondrogenesis. The Piper Hanley group has previously identified that SOX9 becomes ectopically expressed in HSCs during liver fibrosis, and it regulates expression of collagen type 1 (COL1) and osteopontin (OPN); both implicated in pathogenesis of fibrotic disease. Data from global and hepatocyte/biliary epithelial cell Sox9 knockout mice suggest that expression of SOX9 in HSCs is important in regulating the accumulation of ECM and establishing the inflammatory response phenotype in vivo.YAP1 is a downstream effector of the Hippo pathway, which responds to environmental mechanical forces and regulates organ size during development. Liver stiffness is known to rapidly increase during liver fibrosis. In this thesis, YAP1 and signalling partners are shown to localise to the nucleus of HSCs on stiffer substrates. YAP1 is also identified as a regulator of SOX9 and Collagen 1 expression in vitro. The importance of YAP1 signalling in the build-up of ECM during liver fibrosis was confirmed in vivo using a pharmacological inhibitor called Verteporfin.Disruption of the circadian rhythm is linked to many diseases including liver fibrosis. To date most studies relate to changes in metabolic pathways and a pathophysiological role in pro-fibrotic HSCs remains to be investigated. Genetic disruption of circadian homeostasis (ClockDelta19 mice) is shown in this thesis to predispose mice to fibrotic changes in the liver, alter the phenotype of HSCs before injury, and cause more severe fibrosis after chronic injury.This thesis identifies three factors (SOX9, YAP1 and CLOCK) that regulate pro-fibrotic pathways critical for the activation of HSCs and which may be novel targets for the development of anti-fibrotic therapies.