In this article, we scrutinize the concept of ‘urban informality’ in Rio de Janeiro, Brazil. By unpacking key moments in Rio’s history when conceptualizations of informal housing (i.e., favelas) changed, we explain why favelas have been understood in different ways over the last century. Our argument is that the concept of informality, while signaling an important shift in how favelas were understood, also perpetuated orientalist epistemologies in theories of urban development. This helps to explain why the term gained traction when it did, as well as why it remains salient today. In Rio, this means that changing understandings of favelas over the last century reveal little about actual changes within favelas, and more about how different geographic imaginaries were projected onto them, reflecting specific ‘problems’ confronting the city at different moments in history. This is important for seeing how conceptualizations of favelas – including the ways we understand urban informality – tend to mirror a host of latent social and political anxieties connected to urban development, including attempts to govern and control informal space.