Self-assignment, a self-directed method of task allocation in which teams and individuals assign and choose work for themselves, is considered one of the hallmark practices of empowered, self-organizing agile teams. Despite all the benefits it promises, agile software teams do not practice it as regularly as other agile practices such as iteration planning and daily stand-ups, indicating that it is likely not an easy and straightforward practice. There has been very little empirical research on self-assignment. This Grounded Theory study explores how self-assignment works in agile projects. We collected data through interviews with 42 participants representing 28 agile teams from 23 software companies and supplemented these interviews with observations. Based on rigorous application of Grounded Theory analysis procedures such as open, axial, and selective coding, we present a comprehensive grounded theory of making self-assignment work that explains the (a) context and (b) causal conditions that give rise to the need for self-assignment, (c) a set of facilitating conditions that mediate how self-assignment may be enabled, (d) a set of constraining conditions that mediate how self-assignment may be constrained and which are overcome by a set of (e) strategies applied by agile teams, which in turn result in (f) a set of consequences, all in an attempt to make the central phenomenon, self-assignment, work. The findings of this study will help agile practitioners and companies understand different aspects of self-assignment and practice it with confidence regularly as a valuable practice. Additionally, it will help teams already practicing self-assignment to apply strategies to overcome the challenges they face on an everyday basis.