Modern information retrieval (IR) has come to terms with numerous new media in efforts to help people find information in increasingly diverse settings. Among these new media are so-called microblogs. A microblog is a stream of text that is written by an author over time. It comprises many very brief updates that are presented to the microblog's readers in reverse-chronological order. Today, the service called Twitter is the most popular microblogging platform. Although microblogging is increasingly popular, methods for organizing and providing access to microblog data are still new. This review offers an introduction to the problems that face researchers and developers of IR systems in microblog settings. After an overview of microblogs and the behavior surrounding them, the review describes established problems in microblog retrieval, such as entity search and sentiment analysis, and modeling abstractions, such as authority and quality. The review also treats user-created metadata that often appear in microblogs. Because the problem of microblog search is so new, the review concludes with a discussion of particularly pressing research issues yet to be studied in the field.