The aim of the study was to identify factors related to critical incidents that influence the competence of nurse supervisors. Nurse supervisors require considerable competence in order to help supervisees to reflect on their clinical work and to interpret the needs of the patient. A qualitative approach involving the critical incident technique was used. Critical incidents were collected by means of self-reports from 25 nurse supervisors. Two main areas emerged: a professional and a personal stance. The professional stance described the nurse supervisors’ awareness of the importance of creating a secure learning environment and facilitating reflection. The supervisors structured the material and created awareness of fundamental nursing values. The second main area, personal stance, described the nurse supervisors’ behaviour when they gave the participating nurses the opportunity to work through the experiences gained in the daily provision of nursing care. Although they experienced lack of self-assurance during the supervision session, they also expressed security regarding their own performance as nurse supervisors. Nurse supervisors need to include more nursing theory and focus on the nursing process as well as being aware of their own shortcomings and resources. One way for the supervisior to scrutinize his/her actions is to discuss and examine them with a more experienced nurse supervisor colleague.