Professional development is a process starting during undergraduate education and continuing throughout working life. A new nurse's transition from school to work has been described as difficult. This study aims to develop a model describing the professional development of new nurses during their first years of work. To develop this model, constant comparative analyses were performed. The method was a qualitative study of survey data on 330 registered nurses. The results showed that mastering the professional role was the result of an ongoing process building on the nurse's experiences and interactions with the surrounding environment. The professional developmental process involves the following interrelated sub-processes: evaluating and re-evaluating educational experiences, developing professional self-efficacy and developing clinical competence. These sub-processes are influenced by the following factors: social values and norms, healthcare organization, management of new nurses, co-workers, patients and significant others and the nurse's own family and friends. These factors affect professional development directly, indirectly or as mediating influences and can lead to possible outcomes, as new nurses choose to remain in or leave the profession. The results underscore the importance of developing a professional nursing role within the new working context. To facilitate this professional development, new nurses need support from their nursing-school educators and their healthcare employers. The model described here will be the subject of further measurement and testing.