In my dissertation, I study entrepreneur’s relationships to peers and mentors in startup accelerators. Startup accelerators have emerged as popular support mechanism to fledging entrepreneurs providing validation, funding, and resources such as office space and education. However, as a social environment, accelerator research remains underdeveloped, making it difficult to understand how entrepreneurs connect with peer entrepreneurs and mentors within these programs. Addressing this gap, I explore how entrepreneurs form social networks with mentors and peers and how these social networks influence entrepreneurs in return. In Chapter 2, I present a theoretical framework for the development of startup mentoring relationships. Chapter 3 analyzes mentors' willingness to mentor based on signals of relationship quality and entrepreneur’s competence. I show that relationship quality is more important than competence to mentors. Chapter 4 studies the effect of switching to an online accelerator program (due to COVID-19) on the development of peer networks, revealing that networks of online programs are less dense and much more clustered. Finally, Chapter 5 examines the co-evolution of peer networks and entrepreneurial passion within a single accelerator cohort and shows that entrepreneurs select peers based on similarity in passion, and that passion can spill over among entrepreneurs once a social connection exists. My findings have important implications for research concerning entrepreneurial support networks within accelerators and beyond, and provide practical guidance on how to design effective entrepreneurship and mentoring programs that support all participating entrepreneurs.