Objective. Undocumented migration, the phenomenon of living somewhere without legal documents, has replaced traditional immigration to become the fastest growing form of global migration (International Organization for Migration, 2014). Despite burgeoning numbers, this population is often overlooked within psychology (Ellis Chen, 2013). Building on limited literature, this dissertation addresses two primary aspects related to the experiences of undocumented adolescents - ethnic identity development and the acculturation process - from a developmental lens. Paper 1. Although Ellis and Chen (2013) and Maduena (2015) have developed models for ethnic identity amongst undocumented university students, no existing model focuses specifically on the developmental periods of adolescence and emerging adulthood. This qualitative study addresses gaps in the literature by developing an ethnic identity model for undocumented youth. A sample of 23 undocumented youth, between the ages of 16-22, completed a demographic questionnaire and semi-structured interviews. Employing a grounded theory approach, four major themes or stages related to ethnic identity development emerged: Revelation, (Dis)Engagement, Developing Undocumented Consciousness, and Negotiating Undocumented Consciousness. Paper 2. The second paper extends the Tridimensional Model of Acculturation (Ferguson et al., 2013) - whereby individuals acculturate to host, home, and other subcultures within host country - to explore how undocumented adolescents understand and negotiate processes related to acculturation. Using qualitative data gathered from semi-structured interviews with 23 undocumented adolescents, the second paper identified various cultural influences, specific stressors that amplify acculturative stress, and the most critical sites where acculturation occurs. Youth identified attachment to native, host, and subcultures and described school, work, service provision institutions and national borders as the most influential sites of acculturation. Conclusions: This dissertation deepens our understanding of the impacts of undocumented status upon adolescents via an analysis of developmental stages of identity development and an empirical investigation of the process of acculturation. The first paper contributed a new model on undocumented adolescent development and the second paper offered evidence supportive of the Tridimensional Model of Acculturation whereby individuals endorsed acculturating to multiple cultures. Findings provide important insights for service provision, including addressing specific developmental needs of undocumented adolescents by facilitating information sharing to counteract confusion and fear of the unknown. Continued research into processes of identity and acculturation during adolescence may establish best practices for undocumented youth.