Researchers who utilize data from longitudinal surveys nearly always assume that respondents' attributes are not changed as a result of being measured. Yet research in cognitive psychology, political science, and elsewhere suggests that the experience of being interviewed can spark important changes in the way respondents behave, in the attitudes that they possess, and in their willingness or ability to answer questions accurately when they are re-interviewed in subsequent waves. In this dissertation, I evaluate the severity of this problem in longitudinal social science surveys. Using a combination of observational and experimental data, I show that "panel conditioning" has the potential to affect a wide range of attitudinal and behavioral measures, including many items that are commonly used in sociological and demographic research. The causal mechanisms that give rise to these effects are discussed and a large-scale follow-up project is proposed.