In this introductory chapter, written by the six editors of this volume, we introduce and attempt to systematize the key concepts used when discussing online panels. The connection between Internet penetration and the evolution of panels is discussed as are the different types of online panels, their composition, and how they are built. Most online panels do not use probability-based methods, but some do and the differences are discussed. The chapter also describes in some detail the process of joining a panel, answering initial profiling questions, and becoming an active panel member. We discuss the most common sampling techniques, highlighting their strengths and limitations, and touch on techniques to increase representativeness when using a non-probability panel. The variety of incentive methods in current use also is described. Panel maintenance is another key issue, since attrition often is substantial and a panel must be constantly refreshed. Online panels can be used to support a wide range of study designs, some cross-sectional or and others longitudinal, where the same sample members are surveyed multiple times on the same topic. We also discuss industry standards and professional association guidelines for conducting research using online panels. The chapter concludes with a look to the future of online panels and more generally online sampling via means other than classic panels.