The subjects we discuss in this seminar influence major marketing decisions - but are still controversial. Simple, practical assumptions or theories govern most of our decisions in designing, testing and evaluating our marketing activities. But these assumptions are themselves rarely tested and their limitations are almost never spelled out. Our purpose in this seminar is to record where the research industry stands on these issues today. We should start from some common ground. For example, that the effects of marketing activities vary considerably, by execution, across product categories and across brands in these categories. That sales matter more than diagnostics like awareness. That share of category sales is an important objective, but there can be other effects of our activities than on volume share and in the short term. Better, more realistic theories will help us when we assess, after the event, what benefit we got from our marketing investment. We also need theories in order to plan these activities properly, to pre-test them, and to budget for them. The full range of research methods is available to investigate these subjects, from small-scale qualitative work to major numerical analyses. Any solid contribution is welcome. However, we are tackling here what may be the hardest, as well as the most important, of all market research problems.