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MacLane's slogan "adjunction arises everywhere" is widely known, and adjunction has been identified as a key concept (maybe the key concept?) in category theory, eg, in the books by Goldblatt Topoi, Awodey Category Theory, and others:

The notion of adjoint functor applies everything that we’ve learned up to now to unify and subsume all the different universal mapping properties that we have encountered, from free groups to limits to exponentials. But more importantly, it also captures an important mathematical phenomenon that is invisible without the lens of category theory. Indeed, I will make the admittedly provocative claim that adjointness is a concept of fundamental logical and mathematical importance that is not captured elsewhere in mathematics. -- David Ellerman (quoting Awodey) "Adjoints and emergence: applications of a new theory of adjoint functors" Axiomathes 2007

If adjunction, arises everywhere shouldn't we see more examples across the spectrum of maths?

For the most part, it seems the example of natural isomorphism that is most widely quoted is that between the category of vector spaces, and its double dual, as discussed here.

Even a recent book like Roman's Lattices and Ordered Sets only gives 3 examples, including the one above.

The discussion in reply to the question "A bestiary about adjunctions" asked in Math.SSE a year ago primarily revolves around algebraic structures.

Similarly, much of the research seems to be very abstract and algebraic in nature.

But where is application of adjunction and universal mapping property in any of the fundamental theorems or their generalizations (eg, f.t. algebra --> Bezout's theorem). Here is a short list of fundamental theorems in Wikipedia. Or perhaps it's possible in principle but it would take a lot of effort to identify the categories, functors and related constructions (transposition, unit, counit?).

EDIT

Here by the way is a historical timeline - implemented in Mathematica - of advances in adjunctions, motivated by Qiaochu Yuan's choice of Galois theory to address this question:

enter image description here

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    I disagree with the premise of your question. My personal experience has been that there are plenty of examples across the spectrum of math, but I think a list of things called "fundamental theorem of X" is not a representative sample.2012-08-26
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    Seconding Qiaochu Y's comment: things that get promoted to "fundamental theorem" status are not at all necessarily fundamental, nor optimally set in context, etc. Literal interaction of an idea with things so-labeled is not the most interesting criterion. Nevertheless, a broader interpretation of the question is very interesting (even if already often-discussed).2012-08-27
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    @ZhenLin, Mac Lane wrote it in the preface to CWM, page vii. Are you aware of an earlier reference by Lawvere?2012-08-27
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    It's not quite correct to say Joyal and Tierney generalised Grothendieck's _Galois theory_. I'll quote Johnstone's MathReview on their memoir: “The title of this monograph, and much of its terminology, are curiously inept. [...] a better title for this volume would have been "An extension of the descent theory of Grothendieck".” It's also misleading to suggest that Grothendieck introduced Galois theory for covering spaces. The material was known as far back as at least 1951 (Steenrod, _The topology of fiber bundles_, §14), and doubtless was known even earlier. [continued]2012-08-27
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    Rather, what Grothendieck and his school achieved in [SGA 1] was the extension of Galois theory to étale coverings of schemes (a rather more restrictive notion than "covering space" in topology) so as to obtain a definition of "fundamental group" in a setting where there are no sensible notions of paths or loops.2012-08-27
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    @ZhenLin, I'm only responsible for the visualization, not the content; that's likely either from Borceaux & Janelidze "Galois Theories" 2001 or Wille's "Dyadic Mathematics" 2004, and I don't have enough background to interpret most of those sources. Really who can reconstruct the full dependency graph.2012-08-27
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    Note that the original quote for the Axiomathes ariticle is me quoting Steve Awodey's book on category theory.2012-08-28
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    @DavidEllerman, thanks, will edit accordingly. Ps, am planning to post a question re *het morphisms*.2012-08-28

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