This article explores how personal professional theories (PPTs) develop. PPT development of nine junior accountants and nine novice teachers was monitored by repeated measurements over a period of 1.5 years, from the last year of vocational education until the second year of their professional careers. Computer-supported construction of PPT concept maps was used at three moments to test hypotheses derived from theories on expertise development. It could be concluded that on average PPTs became more complex. The PPTs of teachers also became richer, but the generality of PPTs did not significantly increase. Appearance and disappearance of concepts and changes in their importance were observed and appeared dependent on the professional environment of the participants. The findings indicate that: PPT development is an important manifestation of expertise development which could be used to support the professional development of students and beginning workers; and the concept map method can be used to reveal this development.