Background: STEM identity has emerged as an important research topic and a predictor of how youth engage with STEM inside and outside of school. Although there is a growing body of literature in this area, less work has been done specific to engineering, especially in out-of-school learning contexts. Methods: To address this need, we conducted a qualitative investigation of five adolescent youth participating in a four-month afterschool engineering program. The study focused on how participants negotiated engineering-related identities through ongoing interactions with activities, peers, and adults, and the patterns of identity negotiation that emerged across program sessions. Findings: Through the investigation, we developed an "Identity-Frame Model," positing that identity negotiation is an ongoing process of performance and definition work by an individual and recognition and positioning work by other adults and peers that creates emergent, context-specific "identities" and "activity frames" that are made particularly salient during critical identity moments. We also categorized model elements that appeared to be specific to engineering, such as situated identities and activity frames related to failure, collaboration, and competition. Contribution: The study advances the understanding of identity negotiation related to engineering and provides a new framework for investigating situated identity in informal STEM learning contexts.