Transcription factors (TFs) are the master regulators of cellular identity binding to specific DNA sequences to control gene expression. Interestingly, while the human genome encodes around 2,000 TFs it is usually only a set of around five that orchestrate the core transcriptional circuitry defining a cell’s identity. Many of these so-called master regulators are not only essential for the maintenance of a specific cellular state but are also able to impose this cellular state if overexpressed in a different cell type. Somatic cell reprogramming into induced pluripotent stem cells (iPSCs) is arguably the most prominent in vitro cellular conversion event due to the unprecedented therapeutic potential of iPSCs. iPSCs are pluripotent and can hence produce any cell type of the adult organism. Reprogramming into iPSCs is mediated by overexpressing the pluripotency TFs Oct4, Sox2, Klf4 usually in combination with C-Myc. How these reprogramming factors are able to reprogram any cell type into iPSCs is, however, not fully understood. Notably, Oct4, Sox2 and Klf4 are pioneer TFs, which means they are capable of recognising partial motifs exposed on nucleosomes and can hence activate repressed chromatin. Indeed, many of the TFs that have been shown to mediate specific cellular conversion events seem to be pioneer TFs. While this suggests that pioneering capacity plays a key role during TF-mediated cellular conversions it remains largely unknown why this is the case. This project therefore aims at deciphering how Oct4 and Sox2 mediate silencing of the somatic transcriptional circuitry and activation of the pluripotency network during reprogramming. Hence, this work has the potential to not only increase our understanding of somatic cell reprogramming into pluripotency and maintenance thereof but might also identify molecular mechanisms key to other cellular conversion of major relevance to many scientific fields and disciplines.