Microfinance was rapidly hailed as a poverty alleviation tool by development agencies, researchers and practitioners. Despite the increasing capacity of MFIs to manage their financial sustainability, impact studies available report disappointingly low social achievements. Social performance assessment tools available struggle to combat a narrow MFI-centric approach which often overlooks contextual issues and institutional characteristics which can influence MFIs' poverty reduction potential. This research's main objective is to identify which and explain how organisational structures and management systems impact on MFIs' social performance. This work uses a bottom-up research strategy, based on a 10-month extensive fieldwork in Bangladesh, a 490 household data-set, an ethnographic community study in Modhupur and institutional analyses of ASA and PDBF. It analyses the livelihoods, capitals and strategies of rural households in Bangladesh, explores their perceptions and experiences of microfinance and examines the management of socio-financial trade-offs within MFIs at different hierarchical levels.The research's main findings seriously question the poverty reducing potential of standardised commercialised microfinance in settings characterised by vulnerability, shocks and seasonality, such as rural Bangladesh. It finds that although most MFIs have similar poverty reduction missions it is the way in which their organisational structures, managementsystems and working cultures are arranged that shapes their financial and social achievements. There is strong evidence that commercial MFIs can experience a silent practice drift at the field level in Bangladesh and that the commercialisation of MFIs provides strong incentives for the field staff to prioritise the achievement of their financial targets to the detriment of social performance, discouraging them from reporting low social performance.There are therefore few reasons why MFI senior managers should question their model and policies. This drift can manifest itself through malpractices hard-selling of loans, poor client selection and follow-up procedures, forcing clients into borrowing more and larger loans, using extreme forms of pressure through abusive language and behaviours and micro-collateral. This process usually has longer-term negative impacts on clients, especially the very poor who adopt successive short-term coping tactics to meet inflexible repayment schedules. This thesis concludes that commercial microfinance should not be targeted to the poorest and that more consideration should be given to clientselection and follow-up procedures.This thesis argues that the commercialisation of the global microfinance industry serves the interests of diverse stakeholders who contribute to maintaining the industry's reputation though the media. This can be deemed an iceberg industry (that shows little of its actual workings and impacts to the public) which is sustained through considerable support from an increasing number of private investors for whom MFIs' commercial expansion (regardless of its social achievements) serves their financial and political interests.