Definiteness is a semantic feature of noun phrases that distinguishes between entities that are specific and identifiable in a given context, and those which are not. The typical noun phrase picks out a unique, familiar, specific referent. The expression of definiteness varies considerably across languages. Definiteness is usually marked in the English language by the selection of determiners, such as articles. Certain determiners like 'a', 'an', 'many', and 'some', along with numbers mark indefinite noun phrases, while others like 'the', 'that', etc., mark definite noun phrases. Definiteness is also marked morphologically in a few languages. Some languages like Japanese do not express definiteness at all. This book is a compilation of chapters that discuss the most vital concepts and emerging trends in the study of definiteness. Different approaches, evaluations, methodologies and advanced studies on this topic have been included herein. A number of latest researches have been included to keep the readers up-to-date with the global concepts in this area of linguistics.