Panel studies are a particular design of longitudinal study in which the unit of analysis is followed at specified intervals over a long period, often many years. The key feature of panel studies is that they collect repeated measures from the same sample at different points in time. Most panel studies are designed for quantitative analysis and use structured survey data. Panel studies can also use qualitative methods for the data collection and analysis. They may also be constructed from register data, an approach that is common in some countries. This entry concentrates on household panels collected by surveys. Cross-sectional surveys are based on a sample of the population of interest drawn at one time point. In contrast, panel surveys follow the population of interest over an extended time period and are concerned with measuring change over time for the units of analysis within the population. The unit of analysis is typically an individual, but it could also be a firm or a dwelling or any other unit of analysis required by the research design. Panel surveys typically collect data at relatively frequent intervals depending on the design requirements