The Internet has consolidated as a new commercial and marketing channel due to the advance of Information and Communication Technologies and the saturation of conventional media channels. In this context, firms are doing specific (for the Internet channel) promotion strategies to improve its placement on search engines results pages and to increase its website traffic. The objectives of this article are: 1) to provide a context for the Search Engine Marketing (SEM), putting special attention on those actions to optimize the placement in search engines (SEO); 2) to identify and classify what SEO actions firms adopt or could adopt to improve its placement on search engines; and 3) to make an empirical study (centered on Spanish book trades) to quantify how much firms are adopting SEO actions and its effectiveness according to the placement observed in search engines result pages. The results are analyzed, the limitations are discussed and new lines for research are commented.