Are there any good introductory texts on algebraic stacks? I have found some readable halffinsished texts on the net, but the authors always seem to give up before they are finished. I have also browsed through FGA explained (Fantechi et al.). Although I find the level good, it is somewhat incomplete and I would want to see more basic examples. Unfortunately I don't read french.
17 Answers
Anton livetexed notes to Martin's Olsson's course on stacks a few years ago. They are online here. Olsson's notes have been published as:
Algebraic Spaces and Stacks, M. Olsson, AMS Colloquium Publications, volume 62, 2016. ISBN 9781470427986
My general advice is to learn algebraic spaces first. The point is that the new things you need to learn for stacks fall into two categories (which are mostly disjoint): 1) making local, functorial, and nontopological definitions (e.g. what it means for a morphism to be smooth or flat or locally finitely presented) and 2) 2categorical stuff (e.g. what is a 2fiber product). You don't need to do things 2categorically for algebraic spaces, so it makes sense to learn them first. I believe it really clarifies things to learn these separately.
Also, the formal notion of a stack is a generalization of functor. If you are not used to thinking of schemes functorially (e.g. as a functor from rings^op to sets) it will be hard to wrap your head around the notion of a stack. the The intermediate step of learning to think about geometry in terms of functors of points is crucial.
Knutson's book Algebraic Spaces is very good for the EGAstyle content, and its introduction will point you to many nice applications of algebraic spaces that are worth learning and will motivate you to learn the EGAstyle stuff. Laumon and MoretBailly's Champs Algébriques is nice and contains more theorems that just the EGA style stuff.
Its hard to point you any other particular reference without knowing what your goal in learning stacks is.

6$\begingroup$ Stacks generalize sheaves, fibered categories (equivalently pseudofunctors) generalize presheaves (contravariant functors [into Sets]). The key key idea behind stacks is not only the generalization of functors, but also a generalization of glueing. I disagree with the idea that algebraic spaces should be learned first for the following reason: all of the 2categorical "stuff" is equivalent to the 1categorical "stuff" when we restrict ourselves to 1categories. $\endgroup$ Nov 15, 2009 at 6:41

1$\begingroup$ Sorry for the nitpick: a scheme is a covariant functor from commutative rings to sets, not from the opposite category rings^op. $\endgroup$ Dec 31, 2014 at 23:22

3$\begingroup$ It's probably worth pointing out that Olsson's book will be out soon. $\endgroup$– HootMay 9, 2016 at 5:53
Vistoli's notes on descent, grothendieck topologies, fibered categories, and stacks at http://homepage.sns.it/vistoli/descent.pdf are not only just a really good introduction to algebraic stacks, they're some of the best notes I've ever read on any subject. What I really liked is that he took the time to not identify f*g* with (gf)*, which makes the proofs longer, but absolutely rigorous.
He starts with a review of category theory and classical scheme theory, then builds up grothendieck (pre)topologies, then builds up the notion of a fibered category, which is a generalization of a presheaf, then defines stacks in terms of fibered categories and descent. What's really great about this approach is that once you see how fibered categories work, Lurie's approach to higher topos theory ((infty,1)categories generalize categories fibered in groupoids) makes a good deal more sense. I can't recommend it enough.

7$\begingroup$ A nice sequel to that is BarnetLamb minor thesis at people.brandeis.edu/~tbl/minorthesis.pdf The problem with Vistoli's notes is that they are a really good introduction to stacks, but do not mention at all the algebraic stack part. BarnetLamb follows closely Vistoli, so you can just skip many parts with a glance, and treats the problem of what does it mean to be an algebraic stack and why this stuff appears naturally in moduli problems. One word of caution: this is literally filled with typos, so pay attention to what you read. $\endgroup$ Feb 24, 2010 at 12:57

$\begingroup$ It's also worthwhile to point out that Vistoli's (fantastic) notes are nothing more than a corrected version of his contribution (Part 1 = first 4 Chapters) to Fantechi et alli's "FGA Explained", quoted by the OP. $\endgroup$ May 9, 2016 at 3:21
I am not sure if the book I am about to suggest is the halffinished text you are hinting at, but there is a book in progress by Kai Behrend, Brian Conrad, Dan Edidin, William Fulton, Barbara Fantechi, Lothar Göttsche and Andrew Kresch. You can find a link to it here:
It is the most complete reference on algebraic stacks in English that I am aware of. It also has the advantage of being addressed to the beginner.
I think that beyond the basic things, anything deeper you learn about stacks typically involves specific stacks, with certain applications or questions in mind.

$\begingroup$ It's still not published... Just wonder when this great book will be finished... $\endgroup$– WWKFeb 15, 2016 at 16:18

There is an opensource textbook on stacks being created. You can find it here
It's already more then 1400 pages long!

2$\begingroup$ plus. In there is a guide to the literature, which should be helpful. $\endgroup$– GMRAOct 24, 2009 at 0:08

9$\begingroup$ Uhm... 1400 pages for an introductory reference seems a bit too much... $\endgroup$ Dec 24, 2009 at 17:02

1$\begingroup$ Good point :) It is an introduction, and much, much more. It seems to be a good introduction, as well as a good reference book for people who know this stuff well. $\endgroup$– GMRADec 24, 2009 at 17:13

5$\begingroup$ Those were the days...it is counting 6932 pages now! $\endgroup$ Feb 13, 2020 at 21:41
I find the "review of algebraic stacks and Artin's method" in chapter 1 of Faltings, Chai "Degeneration of abelian varieties" very nice.
There was an MSRI summer school on stacks and deformation theory a few years ago. The video of all the talks are online, at the workshop's webpage. There are several copies of notes around, I believe they are on Ravi Vakil's webpage somewhere.
It might not be the best reference for a systematic study of stacks and some of the terminology is old, but Mumford's "Picard Groups of Moduli Problems" (1965) might be a nice complement. It explains why stacks came to be and does a few calculations to show their usefulness.
I always found Algebraic Stacks by Tomas Gomez to be a very readable quick introduction. It is virtually without proofs but explains on 34 pages the most relevant definitions and constructions and discusses the example of vector bundle in some detail. He has both the definition of a stack as a sheaf of groupoids and as a category fibred in groupoids in it.

$\begingroup$ That looks like a nice introduction. The one complaint I have is that the definition of 2functor given in the appendix seems to be rather inconvenient  one usually includes the composition compatibility as part of the datum. $\endgroup$– S. Carnahan ♦Oct 28, 2011 at 18:30
Dennis Gaitsgory is currently running a graduate seminar with a website here. There are quite a few notes and references on there about algebraic stacks. You should first look at the notes from the second and third talks.
I have to follow Alberto's answer with Deligne and Mumford's paper on irreducibility of the moduli of curves.
You might take also a look at this: http://staff.science.uva.nl/~heinloth/SeminarStacks.html
especially the references and more especially the last two paper of them.
Linked below is a note written by Kai Behrend whose first section gives a concise introduction to stacks, building them directly out of (lax) functors from the category of affine schemes.
http://www.math.ubc.ca/~behrend/cet.pdf
(edit: repaired broken link)

1$\begingroup$ I think the introduction is far too concise to be a good introductory reference. $\endgroup$ Feb 24, 2010 at 12:53
Here are some other references:
 Lectures on algebraic stacks (Alberto Canonaco)
 Algebraic Stacks and Moduli of Vector Bundles (Frank Neumann)
 Notes on algebraic stacks (Fredrik Meyer)
Besides the references already given, I like Dan Edidin's
Notes on the construction of the moduli space of curves
https://arxiv.org/abs/math/9805101
I quote :
"In section 3 we return to curves and outline Deligne and Mumford's proof that the stack of stable curves is a smooth and irreducible DeligneMumford stack which is proper over $\operatorname{Spec} \mathbb Z$".
It starts from the basics but still explains a key result in the history of the subject. Highly recommended !
Daniel HalpernLeistner is teaching a foundational course on moduli theory based on stacks at Cornell. Very nice addition to the literature in my opinion.

1$\begingroup$ These web books managed with Gerby look terribly nice. The SP's online book style is definitely the future of written math! Thanks for sharing this link! $\endgroup$ May 8, 2022 at 2:24
there should be a videolecture of Kai Behrend "algebraic stacks" which I once watched. I can no more find it quickly. But i think one could find it on the homepage of Cambridge


$\begingroup$ I meant this youtube.com/watch?v=dAM2v3Zgbsg But of course the video you found and recommand could also be helpful $\endgroup$ Jan 28, 2017 at 17:27