This chapter aims to provide an up‐to‐date overview of research findings on ESL/EFL developmental sequences and stages, and the pedagogical implications of these findings. It opens by defining and exemplifying key terms and surveying previous approaches, focusing on the morpheme order studies and frequency analysis. This section highlights the advances made by these approaches, particularly in the identification of developmental sequences, as well as some of their major limitations, specifically the lack of a clear explanation for sequenced development and a precise definition of acquisition. The next section presents the major, contemporary theory positing developmental stages: Processability Theory (PT) (Pienemann, 1998, 2005). This section shows how, by addressing the shortcomings of previous work, PT offers a coherent explanation of developmental stages and variation within them, as well as a principled criterion of acquisition. This section also presents the developmental stages for ESL morphology and syntax, including the well‐known question stages. The final section explores the rich literature on the practical applications of the ESL developmental stages. By outlining PT's pedagogical suggestions and discussing major criticisms of them, it offers ESL/EFL teachers a time‐tested way of matching pedagogy with the developmental readiness of students.