This book is an essential reference to cutting-edge issues and future directions in information retrieval. Information retrieval (IR) can be defined as the process of representing, managing, searching, retrieving, and presenting information. Good IR involves understanding information needs and interests, developing an effective search technique, system, presentation, distribution and delivery. The increased use of the Web and wider availability of information in this environment led to the development of Web search engines. This change has brought fresh challenges to a wider variety of users' needs, tasks, and types of information. Today, search engines are seen in enterprises, on laptops, in individual websites, in library catalogues, and elsewhere. Information Retrieval: Searching in the 21st Century focuses on core concepts, and current trends in the field. This book focuses on: Information Retrieval Models; User-centred Evaluation of Information Retrieval Systems; Multimedia Resource Discovery; Image Users' Needs and Searching Behaviour; Web Information Retrieval; Mobile Search; Context and Information Retrieval; Text Categorisation and Genre in Information Retrieval; Semantic Search; The Role of Natural Language Processing in Information Retrieval: Search for Meaning and Structure; Cross-language Information Retrieval; and, Performance Issues in Parallel Computing for Information Retrieval. This book is an invaluable reference for graduate students on IR courses or courses in related disciplines (e.g. computer science, information science, human-computer interaction, and knowledge management), academic and industrial researchers, and industrial personnel tracking information search technology developments to understand the business implications. Intermediate-advanced level undergraduate students on IR or related courses will also find this text insightful. Chapters are supplemented with exercises to stimulate further thinking.