The aim of this study was to explore challenging encounters experienced by registered nurses (RN) during their first year in the emergency medical service by using the social learning theory of communities of practice. During the first year in a new professional practice, a new RN experiences a transition during which the new professional identity is being formed. This is a challenging and demanding period of time. According to the learning theory of communities of practice by Lave and Wenger, individuals' learning and development in a new professional practice occurs through participation in social activity and is influenced by context. This study is based on the qualitative data from semi-structured interviews. Thirty-two RNs working in the Swedish emergency medical service were interviewed via telephone during the spring of 2017. A qualitative content analysis with deductive reasoning of the interviews was used. The analysis process generated the main category; New RNs participation is challenged by unpredictability and uncertainty in practice. The main category was based on three generic categories; Loneliness in an unpredictable context, Uncertainty about the team, and Uncertainty in action. The challenges new RNs encounter during the first year relate to all three dimensions of a community of practice; mutual engagement, joint enterprise and shared repertoire. The encountered challenges also relate to the EMS context. Taking into account all these aspects when designing support models for RN's professional development may be advantageous for creating positive development for RNs new to the EMS and/or similar practices.