This article reports on a systematic review of literature on the experiences of substitute teachers, also known as casual or relief teachers. This occupational group are an essential part of school improvement efforts, allowing release time for other teachers to participate in professional learning, complete administrative duties, and attend to personal matters. Although a ubiquitous component of the teaching workforce, little is known about their work conditions, motivations, experiences, and support. This study involved a mixed-methods research synthesis approach. Peer-reviewed studies and dissertations that examined the experiences and needs of substitute teachers in primary/elementary, middle, and secondary/high schools were examined. The study found that, although substitute teachers are a heterogeneous group, their conditions and experiences are less than satisfactory, exacerbated by a lack of targeted support. Education systems need to consider professionalizing this occupational group further. In addition, systems need to develop policies and practices that improve substitute teachers' work.