In this paper we aim at defining vulnerability in a life course perspective. We follow several steps to legitimate and define the concept of vulnerability as a new travelling concept for life course studies. We begin by presenting the social and historical context in which vulnerability emerges as a useful concept. Then we briefly analyse the current scientific production within the social sciences to determine whether vulnerability can be a heuristic concept for interdisciplinary research against other concepts like poverty, poverty and isolation in sociology, depression in psychology or frailty in gerontology for example. Finally, we review existing life course literature (theories of personality development, sociology of stress, accumulation of (dis)advantage) to propose a dynamic view of vulnerability in the long- and short-term and propose to put vulnerability on the research agenda based on the dynamic model of vulnerability, which is developed using the concepts of life trajectory and transition, resources, life events and stressors, and social context.