The word panel describes a continuous collection of identical information from a sample which represents a segment of a population to study. It is by definition permanent and repetitive. There are as many varieties of panels as there are different populations and methods for measuring them. The research objective must determine which population will be studied and the nature of the information collected. For example, we can have doctor panels which measure their prescription orders, user panels which measure specific products or services, measures of the media audience and of course shop panels. In this chapter, we will take a particular interest in those studies generally known as consumer panels. Their objective is to specifically record the everyday purchases of different households. These panels were developed many years ago (since 1948 in the United Kingdom, 1962 in France) in most developed countries and are part of the basic tool kit for professionals working in marketing research and the marketing of mass-consumption products. They are syndicated studies, covering a large number of products and/or services, the results of which are shared between all the players within a given market. This characteristic of covering many customers and many product categories is basic to the very idea of a panel.