This paper is intended to serve as an introduction to a technique that is relatively new to the market research world. It will explain the essential mechanism of pupil change, and describe the equipment and the experimental conditions that are necessary for investigating it. The uses of the technique are then examined, and we present our case for believing that by including it in research programmes of many kinds it can be useful,' illuminating, and in some cases crucial. Experimented findings are also brought forward, which indicate the range of the problems that can be successfully attacked by the use of an Eye Camera. The experience of both Marplan London and Marplan Frankfurt has gone into preparing the paper. This fact underlines an advantage of pupil dilation measurement as a technique that need not recognise international barriers: the standards and conditions for using it will be invariable from country to country. This bypasses language problems, and differences in trading and advertising conditions, as well as varying standards of interviewing and supervision.