As the core contributions of this book address the shortcomings of current web search engines, this chapter gives an introduction to their general architecture and working principles. Interactive and semantic approaches to web search are covered as well. In all cases, it is of utmost importance to return relevant results to users. Therefore, the notion of relevance is specially discussed in the first section. Here, particular challenges in finding relevant information in the web are emphasised. Finally, this chapter explains and motivates the need for a new concept for decentralised web search.