The number needed to treat (NNT) has gained much attention in the past years as a useful way of reporting the results of randomized controlled trials (RCTs). Defined as the reciprocal of the absolute risk reduction (ARR), NNT is the average number of patients needed to be treated to prevent an adverse outcome in one additional patient. This measure has been applied to a number of other research areas involving the development of more sophisticated techniques to calculate and interpret NNTs in medical research. The most important application areas are screening studies, public health research, epidemiology (case–control and cohort studies), crossover studies, studies measuring continuous and survival time data, risk–benefit analyses, and systematic reviews. An overview of the recent developments regarding NNT and a discussion of the advantages, drawbacks, and usefulness of the NNT statistic as a reporting tool are given.