A major challenge for learning theories is to illuminate how particular kinds of learning experiences and environments promote the development of expertise. Research has been conducted into novice-expert differences in various domains, but few studies have examined the processes involved in learners' expertise development. In an attempt to understand the development of expertise in instructional design (ID) among graduate students, this study aimed to investigate (1) the patterns of expertise development among ID learners over an extended period; (2) the roles of expert modelling, peer feedback, self-reflection and participation in a supportive learning community in learners' expertise development; and (3) the interactions of individual differences and the learning environment in learners' expertise development. A qualitative design was used to investigate students' expertise development across a range of dimensions. The participants were two cohorts of 11 graduate students in a program on instructional psychology and design. Data, including observations, interviews, design documents, metacognitive essays and peer feedback, were collected for triangulation and in-depth analysis. The results showed that the two cohorts exhibited similar patterns in their ID expertise development. These development processes were influenced by both self-processes and social influences. Self-processes are determined by the perceptions, motivation and prior knowledge that students bring into the learning environment. Social influences are characterised by a learning community that encourages peer interactions and feedback and is supported by expert modelling and scaffolding.