Sylow theorems for infinite groups Are there classes of infinite groups that admit Sylow subgroups and where the Sylow theorems  are valid?
More precisely, I'm looking for classes of groups $\mathcal{C}$ with the following properties:

*

*$\mathcal{C}$ includes the finite groups

*in $\mathcal{C}$ there is a notion of Sylow subgroups that coincides with the usual one when restricted to finite groups

*Sylow's theorems (or part of them) are valid in $\mathcal{C}$
An example of such a class $\mathcal{C}$ is given by the class of profinite groups.
 A: A number of older papers by V.P. Platonov (in Russian, often followed by English translations) deal with periodic linear groups or linear algebraic groups in which the notions of Sylow theory make sense and where some results from the finite case actually generalize.   One of the more substantial papers deals especially with conjugacy theorems:
The theory of algebraic linear groups and periodic groups. (Russian)
Izv. Akad. Nauk SSSR Ser. Mat. 30 1966 573–620.  
In other papers Platonov also works with classes of topological groups in a similar spirit.
P.S. Concerning sources, the long 1966 paper appears in an English translation (by the group theorist Kurt Hirsch) in volume 69 of the AMS Translations (Series 2), 1969; but this doesn't seem to be accessible online.   There is a Google Scholar entry containing a full text PDF version of the Russian original here.  
A: You may also read Chapter 13 of Kurosh's book Theory of groups, volume 2.
For instance, it contains a proof of Baer's theorem (cited by @Igor) which says that
all p-Sylow subgroups of a locally normal group are isomorphic.
Locally normal means periodic with finite conjugacy classes.
A: Amalgams of finite groups provide another example. Let $A$ and $B$ be finite groups and let $C = A \cap B.$ Suppose that $P$ is a Sylow $p$-subgroup of $A$, and that $C$ contains a Sylow $p$-subgroup of $B.$ Then the amalgam $A*_{C}B$ has a unique conjugacy class of maximal finite $p$-subgroups, but is an infinite group as long as $C$ is proper in both $A$ and $B$. In fact, the process an then  iterated to the case where $A$ and $B$ may themselves be amalgams of finite groups of this type, and so on. For general results on amalgams, see J.-P. Serre's book "Trees". For applications of this type of construction to fusion systems on finite $p$-groups, see two recent papers of mine in Journal of Algebra (Amalgams, blocks, weights, fusion systems and finite simple groups) and Transactions of the AMS (Reduction mod $q$ of fusion system amalgams).
A: Well, the Wikipedia gives an example of a Sylow theorem, and there is more on this in notes Sylow theorems by Igusa. There is also the following paper of Baer:
Sylow theorems for infinite groups
Reinhold Baer
Source: Duke Math. J. Volume 6, Number 3 (1940), 598-614.
A: The best reference for this subject is the book of Martyn Dixon: Locally finite groups and Sylow theory.
A: In groups of finite Morley rank there is a Sylow theory for the prime $p=2$.
