Background: Discussions of maternity care in developing countries tend to emphasise service uptake and overlook choice of provider. Understanding how families choose among health providers is essential to addressing inequitable access to care. Our objectives were to quantify the determinants and choice of maternity care provider in Mumbai's informal urban settlements, and to explore the reasons underlying their choices. Methods: The study was conducted in informal urban communities in eastern Mumbai. We developed regression models using data from a census of married women aged 15-49 to test for associations between maternal characteristics and uptake of care and choice of provider. We then conducted seven focus group discussions and 16 in-depth interviews with purposively selected participants, and used grounded theory methods to examine the reasons for their choices. Results: Three thousand eight hundred forty-eight women who had given birth in the preceding 2 years were interviewed in the census. The odds of institutional prenatal and delivery care increased with education, economic status, and duration of residence in Mumbai, and decreased with parity. Tertiary public hospitals were the commonest site of care, but there was a preference for private hospitals with increasing socio-economic status. Women were more likely to use tertiary public hospitals for delivery if they had fewer children and were Hindu. The odds of delivery in the private sector increased with maternal education, wealth, age, recent arrival in Mumbai, and Muslim faith. Four processes were identified in choosing a health care provider: exploring the options, defining a sphere of access, negotiating autonomy, and protective reasoning. Women seeking a positive health experience and outcome adopted strategies to select the best or most suitable, accessible provider. Conclusions: In Mumbai's informal settlements, institutional maternity care is the norm, except among recent migrants. Poor perceptions of primary public health facilities often cause residents to bypass them in favour of tertiary hospitals or private sector facilities. Families follow a complex selection process, mediated by their ability to mobilise economic and social resources, and a concern for positive experiences of health care and outcomes. Health managers must ensure quality services, a functioning regulatory mechanism, and monitoring of provider behaviour.