Search engine optimization (SEO) is a set of methods aimed at improving the ranking of a website in search engine listings, and could be considered a subset of search engine marketing. The term SEO also refers to "search engine optimizers," an industry of consultants who carry out optimization projects on behalf of clients' sites. Some commentators, and even some SEOs, break down methods used by practitioners into categories such as "white hat SEO" (methods generally approved by search engines, such as building content and improving site quality), or "black hat SEO" (tricks such as cloaking and spamdexing). White hatters say that black hat methods are an attempt to manipulate search rankings unfairly. Black hatters counter that all SEO is an attempt to manipulate rankings, and that the particular methods one uses to rank well are irrelevant. Other SEOs reject the black and white hat dichotomy as over-simplification.

Search engines display different kinds of listings in the search engine results pages (SERPs), including: pay per click advertisements, paid inclusion listings, and organic search results. SEO is primarily concerned with advancing the goals of a website by improving the number and position of its organic search results for a wide variety of relevant keywords. SEO strategies may increase both the number and quality of visitors. Search engine optimization is sometimes offered as a stand-alone service, or as a part of a larger marketing effort, and can often be very effective when incorporated into the initial development and design of a site.

For competitive, high-volume search terms, the cost of pay per click advertising can be substantial. Ranking well in the organic search results can provide the same targeted traffic at a potentially significant savings. Site owners may choose to optimize their sites for organic search, if the cost of optimization is less than the cost of advertising.

Not all sites have identical goals for search optimization. Some sites seek any and all traffic, and may be optimized to rank highly for common search phrases. A broad search optimization strategy can work for a site that has broad interest, such as a periodical, a directory, or site that displays advertising with a CPM revenue model. In contrast, many businesses try to optimize their sites for large numbers of highly specific keywords that indicate readiness to buy. Overly broad search optimization can hinder marketing strategy by generating a large volume of low-quality inquiries that cost money to handle, yet result in little business. Focusing on desirable traffic generates better quality sales leads, resulting in more sales. Search engine optimization can be very effective when used as part of a smart niche marketing strategy.