At first sight, web surveys seem to be an interesting and attractive means of data collection. They provide simple, cheap, and fast access to a large group of potential respondents. However, web surveys are not without methodological problems. Specific groups in the populations are under-represented because they have less access to Internet. Furthermore, recruitment of respondents is often based on self-selection. Both under-coverage and self-selection may lead to biased estimates. This paper describes these methodological problems. It also explores the effect of various correction techniques (adjustment weighting and use of reference surveys). This all leads to the question whether properly design web surveys can be used for data collection. The paper attempts to answer this question. It concludes that under-coverage problems may solve itself in the future, but that self-selection leads to unreliable survey outcomes.