Extensive and repeated testing of a wide variety of alternative reasonable hypotheses is necessary in order to increase knowledge about complex phenomena such as advertising. While non-experimental evidence is useful for less complex issues, laboratory and field experiments, as well as quasi-experimental studies, are needed to obtain useful knowledge about complex ones. Fortunately, much useful empirical research has been conducted on how to create an effective advertisement. A literature review, conducted over 16 years, summarized knowledge from 687 sources that included more than 3,000 studies. The review led to 195 condition-action statements (laws or principles) for advertising. Advertisers often fail to follow these principles, perhaps because they have not previously been available in a codified form. (We were unable to find the principles in a convenience sample of nine advertising textbooks; of the more than 6,500 references in these textbooks, only 24 overlapped with the 687 used to develop the principles.) By using these principles, practitioners can substantially increase advertising effectiveness. There are also opportunities for researchers. Relevant evidence-based papers were published at the rate of 20 per year from 2000 through 2009. The rate of knowledge accumulation could be increased via directed research (e.g., invited papers and business-sponsored research), and by publishing evidence-based research findings on the Internet.