The National Population Policy (2000) aims at complete protection of all children against vaccine preventable diseases by 2010. Urban poor, many residing in slums, comprise about one fourth of India's 285 million urban population. 60% of the children aged 12-23 months in urban India are fully immunized; coverage among urban poor children is a dismal 43%. The inter state variations of immunization coverage in urban areas, reveals a service coverage gap which calls for a rethink on resource allocation and strengthening processes to improve immunization coverage amongst urban poor. Debilitating environmental conditions and high population density in slums expedite disease transmission. Comparisons of urban rural disease incidence indicate a particular urban risk for vaccine preventable diseases. This paper attempts to understand the current scenario and challenges in improving immunization coverage in urban slums; immunization being one of the most successful public health interventions of the past century. It also discusses possible mechanisms for effectively reaching the often left out urban poor. Coordinated activities by the multitude of providers, accurate information based outreach, effective monitoring and community enablement to demand quality services are critical for improving utilization of immunization services by a heterogeneous urban poor population.