Family reunification is a complex process and is consensually considered the best solution for children in care, as soon as the family has changed the dysfunctional patterns that prevent child safety and well-being. Intervention throughout the entire process is crucial to the success of family reunification. This study aimed to explore and understand child protection professionals’ views on factors influencing (un)successful family reunification trajectories. Using a qualitative design, 33 Portuguese child protection professionals participated in five focus groups. The thematic analysis revealed a set of influential factors within three different systemic levels: child, family, and child welfare system. The latter level was clearly predominant, pointing to the powerful role of the intervention as a vehicle for successful family reunification. The results showed the relevance attributed by the professionals to some main intervention guidelines, children–professionals’ relationships, multisystemic assessment and intervention, coordinated work of intervention teams, and sufficient time between the court decision and the child’s re-entry into the family home. The need for early intervention and its continuity after the child’s reintegration into the home also emerged as relevant factors. This study provides in-depth knowledge of professionals’ views on the intervention process, thus contributing to a comprehensive understanding of (un)successful family reunification trajectories.