Working as agents for their patients, health care providers often make treatment decisions on the patient’s behalf. By establishing common standards, clinical guidelines are central to efforts to improve patient care and can expedite the introduction of new technologies. Each year considerable resources are used to disseminate clinical guidelines, though conventional public health interventions often have a limited effect in changing providers’ practice. Using economic theory and methods, research was undertaken to design and evaluate interventions to support the roll-out of malaria rapid diagnostic testing. This thesis contains five research papers on providers’ knowledge, preference and practice in treating patients with malaria symptoms in Cameroon and Nigeria. In this setting, uncomplicated malaria is routinely diagnosed and treated by health workers in outpatient departments and primary health centres, or self-treated using antimalarials purchased at pharmacies and drug stores. Major problems with malaria diagnosis and treatment were identified. Relatively few febrile patients were tested for malaria, many did not receive the recommended antimalarial, and when patients were tested for malaria the test result was often ignored when treatment was prescribed. Moreover, there was no significant relationship between providers’ knowledge and their practice, and preferences over alternative antimalarials were similar among providers working in the same facility or locality. The results of a cluster randomized trial in Cameroon demonstrated that introducing rapid diagnostic tests with enhanced training, which targeted providers’ practice, was more cost-effective than introducing rapid diagnostic tests with basic training, when each was compared to current practice. Since the trial concluded, the Ministry of Health has incorporated the enhanced training in the nationwide roll-out of rapid diagnostic testing. The findings are also relevant for policy makers elsewhere, and highlight the value in developing strategies to improve providers’ adherence to malaria treatment guidelines when expanding access to malaria testing