The research presented is within the context of urban health, and its objectives include: (1) To highlight the efforts made in Geneva in improving access to health care for vulnerable and disadvantaged populations, such as undocumented migrants and detainees; (2) To describe the sociodemographic and epidemiologic profile concerning the main morbidities of these populations; and (3) To outline future developments related to the research, generally concerning access to health care for these vulnerable populations in urban centers. Urban health includes two main aspects. One is the description of the health status of urban populations, and two, an understanding of urban health determinants with the aim to implement those interventions that promote good health. Since 2007 the majority of individuals live in cities, which has a major influence on all spheres of life, especially health. In Europe, the rural/urban transition occurred prior to 1950.1 Urban areas have higher proportions of migrants and other vulnerable populations and concentrate social inequalities. Access to health care for all residents is a fundamental human right. Its realization for vulnerable populations helps to increase social justice thereby serving not only the vulnerable but the entire community. Switzerland's cities have a substantial number of undocumented migrants, i.e., migrants without a legal residency permit. In the beginning of the 1990's they had no structured access to health care. In 1996 the creation of the “Unité Mobile de Soins Communautaires (UMSCO)”, a mobile health care unit attached to the Division of Primary Care of the Geneva University Hospitals (HUG), markedly improved access to health care for undocumented migrants in Geneva. Additionally, this unit improved our epidemiologic knowledge concerning this hard-to-reach and easy-to-miss population. Compared to the urban population with a residency permit, this undocumented population was found to have high rates of unintended pregnancy (odds ratio (OR) 8.0), delayed prenatal care (>12 weeks of amenorrhea) (OR 10.8), increased episodes of violence during pregnancy (OR 8.6), and elevated prevalence of chlamydia (5.8%) or chagas disease (Trypanosoma cruzi prevalence 12.8%). Undocumented Latin American migrants had a greater than fivefold risk (OR 5.5) of TB-related fibrotic signs on chest X-ray. From the perspective of infectious disease control it is particularly important for the urban community to have structures which are in contact with hard-to-reach populations.