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A growing number of routine and research activities, in a wide variety of fields, have the counting of certain types of objects (cells, people, insects, etc.) as one of their main components. In most cases, such counting procedure is performed manually, in a process that is often lengthy and tedious. For that reason, several methods for automatically counting the objects of interest have been proposed in the last two decades. The vast majority of those methods rely on digital images containing the objects to provide an estimate as close as possible to the results manually obtained by human experts. The review is organized in a tutorial-like form, that is, instead of grouping the references according to a given criterion and then describing them, the paper describes some of the main tools and techniques used in this field of research, and then cites the references as sources for additional information and inspiration.