The urban poor, especially those living in peri-urban areas and informal settlements, are particularly exposed to the risk of cascading effects from potential hazards. This study uses a systems thinking approach to validate the hypothesis if CI services are more susceptible to failure and likely to generate severe cascading effects in informal settlements than in formal settlements. Qualitative data on cascading effects of CI failure were collected via interviews and focus group discussions in two hillside settlements, one formal and one informal, in Medellin, Colombia.