What does “success” mean for social reintegration of ex-combatants in urban settings? Although reintegration of former fighters has been at the center of academic and policy discourse for achieving peace, limited attention has been paid to unraveling how social reintegration processes occur and how they are impacted in urban contexts. This thesis studies the specific issue of successful social reintegration in urban contexts and asks why it evidences higher levels of success in some urban settings than others. Drawing on previous research on the impact of security conditions on reintegration processes, it argues that actual or suspected eroded security conditions, caused by the presence of organized armed groups as well as reintegrating ex-combatants whose former unit operated in the same host community, negatively impact levels of success of social reintegration in urban centers because they each increase levels of fear towards ex-combatants among community members. Through process tracing and structured focused comparison, this thesis assesses the explanatory power of the proposed hypotheses for the cases of Bogota and Medellin. Based on the yielded results, support for both hypotheses is identified. Nonetheless given the nuanced conceptualizations propose heiring, future research is called upon to further assess the explanatory power of the proposed models.