In the knowledge economy, many companies are well aware of the vital need to maintain the professional expertise of their workers at a high level. Though there have been a lot of research studies in the areas of professional expertise and workplace learning, few examined the learning pathways novice workers went through to become experts in their professions. Most of the research studies on expertise focused on the nature of expertise and few on its development. As for workplace learning research, most studies focused on the learning resources and methods workers used and the affordances of the workplace in making learning possible. Based on the accounts of expertise development collected from individual information technology (IT) workers in Hong Kong, this paper proposes a four-phase model of professional expertise development in the workplace and discusses its similarities to and differences from extant theories of expertise and skill development.