Using a survey of 1774 users and non-users in 84 slums in three metropolitan cities (Delhi, Ahmedabad and Kolkata), we try to understand the impact of mobiles on their social and economic lives. Urban slum dwellers spend significant amounts on communications, both for a first time acquisition of handset and SIM (nearly 40% of the average household earnings per month), as well as on going expenditure. However, a majority of respondents believe that the use of mobiles has led to an improvement in their economic situation and that these benefits are greater than ownership and usage costs. Mobile also appears to change how slum residents interact with each other. Despite reducing face-to-face interactions, mobile usage is associated with stronger social relationships. In comparing users and non-users, we find differences between users and non-users in terms of income, education and other social characteristics. We also find evidence of hierarchies within households, with women far more likely than men to be only infrequent mobile users or not to have access at all. While cost of a handset is the primary barrier to owning a mobile, non-owners report difficulty in using a mobile, clarity of charges for call-plans and information dissemination as other barriers to ownership.