Panel conditioning refers to the phenomenon whereby respondents’ attitudes, behaviour, reporting of behaviour and/or knowledge are changed by repeated participation in a panel survey. Uncovering such effects, however, is difficult due to three major methodological challenges. First, researchers need to disentangle changes in behaviour and attitudes from changes in the reporting of behaviour and attitudes as panel conditioning may result in both, even at the same time and in opposite directions. Second, the identification of the causal effect of panel participation on the various forms of change mentioned above is complicated as it requires comparisons of panel respondents with control groups of people who have not been interviewed before. Third, other sources of error in (panel) surveys may easily be mistaken for panel conditioning if not properly accounted for. Such error sources are panel attrition, mode effects, and interviewer effects. In this chapter the challenges mentioned above are reviewed in detail and a methodological framework for the analysis of panel conditioning effects is provided by identifying the strengths and weaknesses of the various designs that researchers have developed to address the challenges. The chapter concludes with a discussion of a future research agenda on panel conditioning effects in longitudinal surveys.