This chapter revisits an earlier effort in 1997 to address the ongoing challenge of defining and classifying NGOs. A revised definition of NGOs incorporates faith-based organizations, environmental NGOs and civil society organizations. Once again, organizational attributes are utilized rather than fixed typologies, and two types of NGO descriptors are suggested as the basis of a classification framework. Essential descriptors include orientation (or activities), level of operation (location) as well as a new category of constituency (people). Contingent (or secondary) descriptors include sector/issue focus as well as numerous evaluative attributes that are frequently the focus of discussion about NGOs. The chapter concludes that the NGO sector is vital yet organic, continually evolving in response to shifting social, political and economic realities.