The use of the World Wide Web for survey data collection has been increasing already for a decade. There are several research fields where Web surveys are used regularly because coverage presents a smaller problem and probability sampling is possible – examples are establishment surveys, consumer research of online services, research on students and professionals, etcetera. But web surveys are also regularly used in the field of health research or for market research where researchers must content themselves with non-probability samples. While web surveys are already widely used, survey methodologists still try to show whether they are actually as valid and reliable as more conventional survey methods. One approach to do this is by comparing the results of web surveys to the results of more conventional survey methods such as a mail survey, telephone survey, or face-to-face survey on a particular topic. This invited paper will thus give an overview of the type of research designs used for this purpose, issues addressed (e.g. data quality, measured with differences in unit and item-nonresponse, consistency of answers, substantive answers etc.; cost issues), and results revealed.