This chapter aims to present a brief overview of existing definitions and theorizations of the third sector. It divides them into two main categories. The first puts the emphasis on definitions, while the second comprises approaches that theorize. First, those who aim to define the third sector seek to understand it as a certain kind of institution (or group of actors) with specific “third sector” characteristics. These “ontological” definitions of the third sector offer differing views on what it is made up of and what is excluded from this category. This category can be subdivided into an American and a European view (though no neat geographical division exists). The former sees the third sector as a discrete sector characterized by certain qualities such as civility, whereas European theorists tend to take “the hybrid view” that views third sector organizations essentially as mixtures of other kinds of social organization such as private and public, or hierarchic and anarchic. The second approach conceives of the third sector not as an object out there waiting to be authoritatively defined but as a kind of societal process. These “epistemological” approaches (so-called because they look at the kinds of knowledge that they depend upon) include a variety of perspectives too. A systems theory view sees the third sector as a particular form of communication between different societal systems facilitating certain activities while obstructing others. Discourse-theoretical accounts view it as a form of ordering of people and ideas (e.g., a “governmentality”). Finally, a critical communicative civil society view of it as a zone of dialogue or struggle between diverse actors and holders of institutional power.