The concept of informality has often been misunderstood. Its, especially, urban character is rarely appreciated. With respect to urban development, it was once thought to distinguish the Global South, a view that is changing, but an alternative has not been systematically articulated. Based on an extensive survey of literatures, this article clarifies the urban character of informality, articulates its global relevance, and proposes a framework for understanding its major modes: latent, diffuse, embedded, overt, and dominant. Their relative frequency distinguishes cities, regions, and nations while their identification facilitates the comparison of places.