This document discusses legal issues surrounding the purchase and use of trademarks as keywords in search engine advertising in the United States and European Union. In the US, search engines are generally not liable for trademark infringement for offering trademarks as keywords, but advertisers could be liable if their ads using trademark keywords cause likelihood of consumer confusion. In the EU, both search engines and advertisers could potentially be liable, but liability depends on whether the ad text itself causes confusion about the trademark owner's connection to the ad. The document provides recommendations to avoid liability, such as not using trademarks in ad text and not misleading users about the source or affiliation of the goods or services.