In the Norwegian Military Doctrine the need for a joint professional identity is emphasized. This longitudinal study examines military professional identity among a sample of 234 students at Norwegian Officer Candidate Schools. The students at the Candidate Schools belong to one of three different branches, fulfilling different functions, an operative branch, a technical branch or a logistic branch. The aim of this study is to investigate whether differences in professional military identity exist between students affiliated with different branches. A survey measuring military professional identity was administered on two occasions, during admission to the schools and one year later. Military professional identity is conceptualized, operationalized and measured along three identity dimensions: Professionalism, Individualism and Idealism. Professionalism is an attempted operationalization of the joint professional identity described in the Doctrine. The analysis revealed a significant increase in Individualism within all three branches. On the contrary, only students belonging to the operative branch showed a significant increase in Professionalism. The social identity approach was applied to gain an understanding of the processes involved when an individual is socialized into Officer Candidate Schools.