In this approach, the definition of IC 50 is based on the assumptions that there is: (a) a monotonic relationship between the dose of the compound tested and the response in the assay and, (b) a consistent 50 % response can be defined clearly. The sigmoid E max model is comprised of four parameters: minimum and maximum effects, steepness (=slope) of the dose–response curve, and concentration of the drug required to inhibit growth to 50 % of that observed in the absence of the drug (IC 50). Standard analysis of in vitro (culture-adapted Plasmodium strains) and ex vivo (fresh Plasmodium clinical isolates) assay data is commonly conducted by using non-linear regression to fit a sigmoid E max model to each sample’s concentration-effect data. Although these assays are highly informative, the comparison of data between laboratories or field sites is often problematic as numerous variations exist between protocols, such as the initial parasitaemia, incubation time, culture haematocrit, and the use of alternative media and supplements. In most, the parasites’ drug susceptibility is defined by measuring growth (i.e., schizont maturation) or replication (i.e., re-invasion assays) in the presence of varying concentrations of anti-malarial compounds. Ī variety of assays are available to measure drug susceptibility in Plasmodium falciparum. In addition, in vitro assays enable the measurement of drug sensitivity without the confounding effects of clinical efficacy such as host immunity and the pharmacokinetics of the drug. In vitro assays also provide an opportunity to assess drug susceptibility of parasites to individual drugs, thereby allowing preventive measures to be taken before clinical treatment failure occurs. Although artemisinin-based combination therapy (ACT) has been implemented widely for the treatment of falciparum malaria and has proven to be beneficial, it is important to consider that resistance to one component of the therapy can be masked by a partner drug which retains high anti-malarial efficacy. In vitro assays for assessing anti-malarial drug susceptibility are an important part of monitoring drug resistance and investigation of novel anti-malarial compounds. Resistance has emerged and spread to all currently available anti-malarials and reinforces the need for better surveillance strategies. Efforts to control and eliminate malaria have failed repeatedly, often due to the spread of drug-resistant parasites and vectors. Malaria remains a serious public health problem in endemic countries.