Web-based enrollment in surveys and studies is increasingly attractive as the Internet is approaching near-universal coverage and the attitude of respondents toward participation in classical modes of study deteriorates. Follow-up is also facilitated by the web-based approach. However, the consequent self-selection raises the question of the importance of representativity when attempting to generalize the results of a study beyond the context in which they were obtained, particularly under effect heterogeneity. Our review is divided into three main components: first, sample surveys or prevalence studies, assessing the frequency or prevalence of some attitude or disease condition in a population from its frequency in a sample from this population; second, generalization of the results from randomized trials to the population in which they were performed and to other populations; and third, generalization of results from observational studies.