The early stages of an academic career are fraught with insecurity. By focusing on the individual and his or her background, this article sets out to analyse and develop theories for this insecurity. We see academic career insecurity as a mix of how much someone wants to pursue a job in academia and what they feel is the probability of reaching their goal. The article draws on concepts of boundaryless careers and protean careers to theorise about the antecedents of insecurity. Empirical analysis is based on survey data from early-career researchers at a large Austrian university. The findings indicate that the most important individual factors that reduce academic career insecurity are the willingness to be geographically mobile, self-attribution of previous career success, a high proportion of working time devoted to research and networking, as well as being at an advanced career stage. The article demonstrates the potential and limits of the boundaryless and protean career concepts for studying academic careers. Practical measures are that universities should provide early-career researchers with temporal space for research and networking, facilitate stays at other universities, inform them about career success factors, and tailor faculty development programmes to the distinct stages of academic careers.