With apologies to the late Edward Gorey. I blame aleffert and chrisamaphone :) Categorey Theory A is for Adjoints, they're important, I hear, B is for Bicat, where 2-cells cohere. C is for Colimit, a quotient of sum, D is for Diagram, chasing them's fun. E is for Eilenberg, of axiom fame, F is for Functor? didn't catch her name... G is for Groupoid, where you can always undo, H is for Homset, and Homology, too. I's for Identities, lacking complex antics, J's for Joyal, who made up some semantics. K is for Kan, extensions left and right, L's for Lawvere, his calculus a delight. M's for MacLane — without him where'd we be? N's for the Nose, on which things agree. O is for Operads, algebras of trees, P is for Pointless, like some topologies. Q is for Quantales, locales in disguise, R is for Regular, when kernels coequalize. S is for Sets, in which most still believe, T is for Topoi, emerging from sheaves. U's for Underlying, scrapping useless fluff, V's for V-category, think "homsets with stuff ". W's for this Week, during which Baez finds, X's for eXcision, with cycles in mind, Y's for Yoneda, his lemma beneficial, Z is for Zero, both final and initial.