The concept of inreducible complexity is central to the origins controversy. Ineducible complexity (IC) may be defined as any machine or system that requires two or more parts in order to function. Examples range from molecules to mousetraps, organelles, and organisms such as humans. This essay explores the relationship between IC and complexity, clarifying the levels of IC such as the irreducible core and its mode of function. IC has been used in a wide variety of disciplines for over a (...) century. Objections to irreducibte complexity include co-option, the "junk DNA" theory, and the scaffolding argument. Co-option, a major argument against IC, attempts to explain how IC can be achieved through natural means by utilizing existing parts to construct a new biological machine or structure. Yet examples of IC in both the biological and non-biological wortds show that such common objections do not invalidate the concept. IC is firmly established. (shrink)