In low income countries the majority of health care is sought in the private sector, often through drug retailers, but little information is available on retail competition and regulation. This thesis addresses this gap, in the context of the market for fever and malaria treatment in rural Tanzania, investigating how market structure, provider conduct, regulation and consumer demand influence treatment outcomes. Data were collected in three districts, where the main treatment providers were public and private health facilities, drug stores and general shops. Following a census of retailers, data on supply were collected from facilities and shops through in-depth interviews, a structured survey, and retail audits. Data on demand were collected through a household survey. Analysis focused on retailers, supplemented by data on facility services and consumer demand, where this was central to an understanding of retailer behaviour. Retailers were an important source of fever/malaria treatment, with the majority of retail antimalarial sales occurring through drug stores. Retail providers increased the accessibility, range and reliability of drug stocks, but several market failures were evident. Market concentration was high, price competition was weak, information on treatment quality was poor, and negative externalities arose from inappropriate drug use. These failures contributed to low antimalarial coverage, use of ineffective antimalarials, under-dosing, and inequitable access to quality care. Government failures were also evident, in the form of poor quality public sector treatment, and inadequately implemented regulation. To optimise the planned introduction of antimalarial combination therapy, public facility care must be improved. However, facility-only provision will not improve treatment for the majority of fever/malaria visits, which are likely to remain to shops. Implications for widening combination therapy provision to the retail sector are outlined, including the selection of appropriate retailers, maintenance of affordable prices, effective communication with consumers and providers, and a constructive role for regulation.