Background: Reunification with birth families is often a primary goal for children placed in foster care, yet the decision-making processes surrounding reunification remain complex and multifaceted. Objective: This systematic review aims to identify and analyze factors influencing the reunification of children and youth with their birth families after a foster care placement. Method: Four databases (PsycINFO, Scopus, Web of Science and PubMed) were searched, resulting in 5309 articles. Thirty-four studies were selected based on inclusion criteria, all providing multivariate quantitative results of factors related to reunification or time to reunification, including child-related, family-related, case-related, and systemic factors. The studies, primarily conducted in the United States (n = 23), involved over one million children under 18 years old who experienced foster care. Most studies used administrative data (n = 24), with sample sizes ranging from 125 to 275,774 participants. Results: The review highlights that various factors influence reunification outcomes. Older age at entry, family stability, and foster carer involvement increase the probability of reunification, while health issues, poverty, and parental problems reduce it. Conclusions: Recommendations are made for practice, policy, and future research to improve reunification outcomes. Key areas include supporting families, training foster carers, conducting early screenings, prioritizing equity in policy, addressing disparities, strengthening post-reunification services, and focusing future research on underrepresented populations, longitudinal studies, organizational factors, and challenges such as neglect and abandonment.