Doctors leave their practices to take up positions in government ministries, the pharmaceutical sector, the media, or electoral mandates; artists work for administrative inspectorates; attorneys become consultants... The social science studies in this book centre on a well-known phenomenon—social players changing employment or professional activities—also known as “second careers”, “vocational retraining”, or even “political” or “intellectual involvement”.The book suggests redefining these shifts in activity as professional “travel” or “border crossing,” which entail taking on or giving up “positions” and require mobilising different and more or less convertible types of “capital.” The goal is to reevaluate the current concerns surrounding professional mobility through case studies involving doctors, attorneys, farmers, police officers, senior civil servants, consultants, and public servants.