Purpose: This paper aims to examine the development of expertise in an applied discipline by addressing the research question: How is professional expertise developed in an applied profession? Design/methodology/approach: Using a grounded theory methodology (GTM), nine technical-tax experts, and three experienced, non-expert tax professionals were interviewed regarding their experience in developing technical-tax expertise. Using GTM procedures, a core concept and variables (categories and properties of categories) were identified. A theory is advanced which explains the interaction of the core concept and the variables. Findings:This theory proposes that expertise in applied disciplines occurs through years of engaging in the high-value, non-routine work. Professionals with an intelligence matched to the discipline and willingness to work hard are more likely to be successful in this non-routine work. Professionals who find the discipline fascinating and who revel in ambiguity are likely to repeatedly seek this non-routine work. Finally, professionals in organizations with complex client issues are more likely to have opportunities to engage in non-routine work. Research limitations/implications: This study proposed a theory related to a very specific profession--tax accounting. Future research would be appropriate to determine whether other applied disciplines have a similar dynamic in developing expertise. Originality/value: Based on existing theories of expertise, this study developed a new theory of how professional expertise is developed in an applied discipline.