This chapter examines how issues of development International Non-Governmental Organisations (INGOs) are conceptualised broadly, and then their conceptualisations within international relations and within development studies. It focuses on large-scale International Non-Governmental Organisations working in international development and humanitarian relief. The chapter describes how development studies theorising of INGOs, with its more functional analysis of these organisations and the work they do, has been able to better capture how INGOs have been influential not only in their advocacy work but also the influence they have in their service provision work. Critical development theories are inclusive of key development issues, such as the role of gender, environmentalism, culture and class. Thus, while some international relations theories, namely constructivist theories, are more inclusive of INGOs, the majority of international relations theories either do not take them into account or are very focused on particular functional elements of INGO work.