We still cannot look inside people’s heads. To measure emotions, opinions, evaluations, associations, and intentions, we have to ask people about these phenomena. Our data collection has changed over time, and many different methods are available. At present, Internet surveys are widely used in market research and are becoming an important data collection tool in universities. The main advantages of Internet surveys are the speed and cost reduction in comparison to more traditional data collection methods, the positive effect of self-administration, such as more privacy, use of more complex questionnaire routing, and in general a greater interactivity. Disadvantages are problems of Internet coverage of the general population, especially under-coverage of certain sub-groups such as the elderly and the lower educated, and non-response. This contribution provides a review of existing studies comparing online surveys with more traditional data collection methods