Artemisinin-based combination therapies (ACTs) are the first-line treatments for Plasmodium falciparum malaria worldwide. In Western Cambodia,artemisinin resistance has been defined as a long half-life of parasite clearance (T1/2) in response to an artemisinin, given orally for uncomplicated malaria. We hypothesize that this artemisinin resistance phenotype compromises the efficacy of ACTs. The primary objective of this study is to compare P. falciparum recrudescence rates in Western, Northern and Eastern Cambodia, following dihydroartemisinin-piperaquine (DHA-PPQ) treatment. The secondary objective of this study is to determine whether parasite recrudescence is associated with long T1/2. In the Parasite Recrudescence Study, patients with uncomplicated malaria will receive directly-observed treatment with DHA-PPQ over 3 days. We will follow these patients weekly for 9 weeks to identify those with recurrent parasitemia and use genotyping methods to distinguish recrudescences from reinfections. We will enroll a subset of these patients who have an initial parasite density greater than or equal to 10,000/microL in the Parasite Clearance Rate Study and follow their parasite densities more intensively by examining 6-hourly finger prick blood samples until parasites are undetectable by microscopy. With these data we will calculate and compare T1/2 values from patients with and without recrudescent parasitemia. Using various laboratory assays, we will also explore the contribution of host factors to T1/2 variation, and whether naturally-acquired immunity reduces the risk of drug-resistant parasite recrudescence.