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Forgetting or ignoring category theory (I hardly knew any in 1971), as Todd says I used monads concretely and constructively to manufacture spaces Y such that $\Omega ^n Y$ is equivalent to $X$, where $X$ is a space with an action by a suitable operad. Operads encode lots of operations, and their associated monads coalesce all those operations into a single operation, the product of the monad. (The portmanteau word operad combines operation and monad, inspired by this connection and by Lewis Carroll). This concrete coalescence of information is the essence of many, but by no means all, applications of monads. In iterated loop space theory, the combinatorial monads associated to operads mesh with the more abstract monads $\Omega^n\Sigma^n$ associated to the adjoint pair of functors $(\Sigma^n,\Omega^n)$.