The notion of fallibilism has acquired many different senses in contemporary epistemology and the philosophy of science. In this propaedeutic chapter, I endeavour to establish some order in this motley landscape so that we can make appropriate distinctions and establish useful relationships between the different uses of the term. I discuss fallibilism in relation to (1) the human cognitive capacities and instruments (both material and conceptual) used to understand the world, (2) beliefs and knowledge claims, (3) justifications and reasons, and (4) the meta-knowledge that we possess knowledge. Having distinguished these four general senses, I advance further divisions within some of them and important relationships between them. This work is useful for the subsequent chapters.