This paper is in three parts. Part 1 introduces and describes the Strong and Weak theories. Outside observers (both protagonists and antagonists of advertising) and many practitioners believe in the Strong theory. This paper introduces facts suggesting the likelihood that the Strong theory is not universally or even generally valid. The paper hypothesizes that for "high involvement" products and for the small proportion of successful new brands in "low involvement" product fields, advertising works according to the Strong theory. By contrast, in the cases of established brands in "low involvement" product fields, advertising is more likely to operate according to the Weak theory. Taking account of the large but unquantified amount of advertising that has no effect at all, it is estimated very approximately that more than three-quarters of all advertising works (or fails to work) according to the Weak theory. Part II argues that the almost universal belief in the Strong theory has contributed to considerable waste. American universities are imbued with the Strong theory, so that most new entrants into the advertising business are indoctrinated with it. The resultant attitude and practice of over-promise and under-delivery have had malevolent effects. Part III argues that research has an important role in any evaluation of the Strong and Weak theories. Case-by-case study depends on our ability to relate advertising exposure to a longitudinal tracking of penetration and purchase frequency. This is a practicable albeit difficult procedure. A final point relates to advertising research. Quantitative copy testing employing measurements of intrusiveness, recall and persuasion - procedures widely practiced in the United States - is predicated on the implicit assumption that advertising is working according to the Strong theory. Qualitative creative development research (strongly associated with London agencies) is more harmonious with the Weak theory: and is often flexible enough also to give fair treatment to advertising that works as a strong force.