Finding information about entities is the most common information needs when we do search on the Web. While some search engines can recognize specific types of entities, true entity-oriented search still has a long way to go because of high ambiguities in names across documents. This book introduces the entity linking task and selects the identification of gene mentions in biomedical articles as a case study to explore the challenges. Unlike most previous works, we consider the task from the instance-based level and introduce a collective approach to deal with those challenges by using both contextual information of each individual instance and relations among them.