This article discusses the methodological challenges of legislative surveys. Following an overview of different types of survey biases, the article argues that self-selection and misreporting are the most critical problems for legislative surveys. In order to identify the self-selection and misreporting biases, we compare the answers to a survey from Swiss members of parliament with their observed behavior in the parliament. The empirical analysis shows that the survey sample has a substantial misreporting bias. We conclude that the parliamentarian's attitude towards the survey object is strongly linked to their response. These issues should be addressed to improve the design and quality of legislative surveys.