Over one billion people live in the world’s 200,000 slums and informal settlements. We used data generated from mobile phones to better understand one of the largest slums, Kibera located in Nairobi, Kenya. Using call logs from June 2008 June 2009 and theories from human geography, economics, sociology, journalists, and anthropologists as a basis, we tested the validity of a few prominent theories. In particular, we focused our research on migration patterns out of Kibera, inferring places of work, and tribal affiliations.