Clean URLs, also sometimes referred to as RESTful URLs, user-friendly URLs, or search engine-friendly URLs, are Uniform Resource Locators (URLs) intended to improve the usability and accessibility of a website or web service by being immediately and intuitively meaningful to non-expert users. Such URL schemes tend to reflect the conceptual structure of a collection of information and decouple the user interface from a server’s internal representation of information. Other reasons for using clean URLs include search engine optimization (SEO),conforming to the representational state transfer (REST) style of software architecture, and ensuring that individual web resources remain consistently at the same URL. This makes the World Wide Web a more stable and useful system, and allows more durable and reliable bookmarking of web resources.

Clean URLs also do not contain implementation details of the underlying web application. This carries the benefit of reducing the difficulty of changing the implementation of the resource at a later date. For example, many URLs include the filename of a server-side script, such as , or. If the underlying implementation of a resource is changed, such URLs would need to change along with it.

Likewise, when URLs are not “clean”, if the site database is moved or restructured it has the potential to cause broken links, both internally and from external sites, the latter of which can lead to removal from search engine listings. The use of clean URLs presents a consistent location for resources to user-agents regardless of internal structure. A further potential benefit to the use of clean URLs is that the concealment of internal server or application information can improve the security of a system. A URL will often comprise a path, script name, and query string. The query string parameters dictate the content to show on the page, and frequently include information opaque or irrelevant to users—such as internal numeric identifiers for values in a database, illegibly-encoded data, session IDs, implementation details, and so on.

Clean URLs, by contrast, contain only the path of a resource, in a hierarchy that reflects some logical structure that users can easily interpret and manipulate. The implementation of clean URLs involves URL mapping via pattern matching or transparent rewriting techniques. As this usually takes place on the server side, the clean URL is often the only form seen by the user. For search engine optimization purposes, web developers often take this opportunity to include relevant keywords in the URL and remove irrelevant words. Common words that are removed include articles and conjunctions, while descriptive keywords are added to increase user-friendliness and improve search engine rankings.

A fragment identifier can be included at the end of a clean URL for references within a page, and need not be user-readable. Some systems define a slug as the part of a URL that identifies a page in human-readable keywords. It is usually the end part of the URL, which can be interpreted as the name of the resource, similar to the basename in a filename or the title of a page. The name is based on the use of the word slug in the news media to indicate a short name given to an article for internal use. Slugs are typically generated automatically from a page title but can also be entered or altered manually, so that while the page title remains designed for display and human readability, its slug may be optimized for brevity or for consumption by search engines. Long page titles may also be truncated to keep the final URL to a reasonable length.

Slugs may be entirely lowercase, with accented characters replaced by letters from the English alphabet and whitespace characters replaced by a dash or an underscore to avoid being encoded. Punctuation marks are generally removed, and some also remove short, common words such as conjunctions. For example, the original title This, That, and the Other! An Outré Collection would have a generated slug of. .