  Web archives are a historically valuable source of information. In some
respects, web archives are the only record of the evolution of human society in
the last two decades. They preserve a mix of personal and collective memories,
the importance of which tends to grow as they age. However, the value of web
archives depends on their users being able to search and access the information
they require in efficient and effective ways. Without the possibility of
exploring and exploiting the archived contents, web archives are useless. Web
archive access functionalities range from basic browsing to advanced search and
analytical services, accessed through user-friendly interfaces. Full-text and
URL search have become the predominant and preferred forms of information
discovery in web archives, fulfilling user needs and supporting search APIs
that feed complex applications. Both full-text and URL search are based on the
technology developed for modern web search engines, since the Web is the main
resource targeted by both systems. However, while web search engines enable
searching over the most recent web snapshot, web archives enable searching over
multiple snapshots from the past. This means that web archives have to deal
with a temporal dimension that is the cause of new challenges and
opportunities, discussed throughout this chapter.
