Introduction Human factors, such as teamwork and communication, have an important impact on patient safety in hospitals. Hospitals have a patient safety problem, with more than every 10th patient experiencing errors and adverse events during their hospital stay. Root cause analysis have showed that ineffective teamwork and communication failures are the most common causes of errors and adverse events. To improve patient safety in hospital units, healthcare professionals needs competency in teamwork, such as communication, decision making, leadership, situational monitoring and mutual support. Interprofessional team training is a key strategy for improving teamwork and patient safety in hospital units. Previous research on interprofessional team training in specialty units has showed promising results; however, the impact on surgical wards is uncertain. The Team Strategies and Tools to Enhance Performance and Patient Safety (TeamSTEPPS) program had not been implemented in Norway. Team decisionmaking has not yet been studied previously among multiple healthcare professionals across diverse hospital units. Aim The overall aim of the thesis is to gain knowledge about teamwork in hospital units and to evaluate and explore the impact of an interprofessional team training intervention regarding teamwork and patient safety culture in a surgical ward. The specific aims of the sub-studies are as follows: 1) To translate the CSACD-T questionnaire into Norwegian and test it for psychometry properties. The further aim is to describe and compare healthcare personnel's perceptions of collaboration and satisfaction about team decision-making across hospital units (Study I, paper 1). 2) To evaluate the professional and organizational outcomes of an interprofessional teamwork intervention among healthcare professionals in a surgical ward after 6 and 12 months (Study II, paper 2). 3) To explore if an interprofessional teamwork intervention in a surgical ward changes the healthcare personnel’s perceptions of patient safety culture, perceptions of teamwork, and attitudes toward teamwork over 12 months (Study II, paper 3). Methods Study I (paper 1) had a cross-sectional design. The Collaboration and Satisfaction About Care Decisions in teams (CSACD-T) questionnaire was used for the survey conducted among healthcare professionals across multiple hospital units (hospital A and B). Study II (paper 2) used a pre-post design with re-measurement (hospital C), with surveys (CSACD-T, TeamSTEPPS Teamwork Perceptions Questionnaire (T-TPQ), and Hospital Survey of Patient Safety Culture (HSOPS)) distributed to healthcare professionals in the intervention ward at baseline and after 6 and 12 months. Study II (paper 3) used a controlled quasi-experimental design, with surveys (CSACD-T, T-TPQ, HSOPS, and TeamSTEPPS Teamwork Attitude Questionnaire (T-TAQ)) distributed to all healthcare professionals in the intervention ward and control ward (hospitals C and D) at baseline and after 12 months. The intervention was a 6-hour TeamSTEPPS interprofessional team training included simulation training, followed by implementation of teamwork tools and strategies in the ward over 12 months (hospital C). The implementation followed Kotter’s eight steps for leading change. The human factors systems engineering initiative for patient safety (SEIPS) model was used as a theoretical perspective. Results The CSACD-T questionnaire showed promising psychometric properties in terms of construct validity and internal consistency. The scores of collaboration and satisfaction with care decisions in teams varied among unit types and were highest among the healthcare professionals in the wards, with a significant difference between the maternity ward and emergency room (paper I). The outcomes from the intervention study showed significant changes in organizational outcomes after six months, and were in the following areas of patient safety culture: “Organizational Learning and Continuous Improvement” and “Communication Openness” (paper 2). After 12 months, significant changes were found in professional outcomes within the intervention ward, which were in three perceptions of teamwork dimensions: “Situation Monitoring,” “Mutual Support,” and “Communication”, in addition to organizational outcomes, which were in three patient safety culture dimensions: “Communication Openness,” “Teamwork Within Unit,” and “Manager’s Expectations & Actions Promoting Patient Safety” (papers 2 and 3). The improved teamwork dimension “Mutual Support” was found to be a predictor of “Patient Safety Grade” (paper 2). The controlled results revealed significant differences favoring the intervention ward in three patient safety culture measures: “Teamwork Within Unit,” “Overall Perceptions of Patient Safety” and “Patient Safety Grade” (paper 3). Conclusion This thesis presents new insights into team decision-making in diverse hospital units, as reported by healthcare professionals from multiple healthcare professions. The thesis also presents new insights into the impact of an interprofessional TeamSTEPPS intervention in a surgical ward in Norway. The outcomes are promising, indicating that TeamSTEPPS intervention improves teamwork and patient safety culture in a surgical ward. The causal relationships among inputs, processes, and outcomes are, however, not certain, and further studies are required to confirm the outcomes of this comprehensive and well-described interprofessional team-training intervention. Nevertheless, the knowledge from this thesis adds to the vast field of teamwork and patient safety research internationally.