I want to include a list of related equations, say, for a proof, in my LaTeX document. As far as I know, I have two good options, eqnarray and align.
What is the difference between eqnarray and align, and how do I know which I should be using? Or does it matter at all?
Although eqnarray may seem to work "well enough", Avoid eqnarray! Avoid eqnarray! Avoid eqnarray!
Two main problems are mentioned in the doc above:
eqnarray sets horizontal space around the = operator that is not consistent with the space set in other environments, such as \[...\] or $$...$$ (it is wider).eqnarray (also eqnarray* !) has an ill-defined equation numbering, which leads to numbering errors on referencing—mostly when using the command \nonumberUse align and the rest of the ams environments. See texdoc amsldoc (PDF) or the Short Math Guide for LaTeX for documentation on how to use them.
eqnarray" has been updated and published in tugboat 33:1. this will be accessible only to tug members until spring 2013, but after that it will be open to anyone. eqnarray (unless for some reason you can't use the amsmath ones). eqnarray produces spacing that is inconsistent in these three ways" would be helpful. Some of the other answers make reference to spacing problems, but no answers here clearly summarize the spacing problems. align is from amsmath, while eqnarray is from base LaTeX, so I would expect the former to be better. Some differences:
eqnarray has two alignment points (it's basically just array with a default preamble); align has one. x + y &=& z versus x + y &= zeqnarray changes the spacing at the alignment points depending on different factors; align keeps it fixed (which is generally what you want)eqnarray allows page breaks between lines; align doesn't\\ * is treated the same as \\* in eqnarray, but won't work in align (since * shows up commonly in equations)(largely from The LaTeX Companion §8.2.1)
align allows page breaks between lines with the global declaration \allowdisplaybreaks. Besides the better spacing and the fewer ampersands to type, a big advantage of align vs eqnarray is that you can include a \qedhere at the end of the last line and have the nice CQFD square (also called a “Halmos”) placed at the same height as your last formula, and not underneath.
\begin{proof} The proof is a follows: \begin{align} (x+y)^3&=(x+y)(x+y)^2\\ &=(x+y)(x^2+2xy+y^2)\\ &=x^3+3x^2y+3xy^3+x^3.\qedhere \end{align} \end{proof} 
The align environment only works if you use the AMS (American Mathematical Society) packages. If you need to use journal specific document classes or style files, the align environment may not be available. (For example, when I needed to use the iopart class for submission to an Institute of Physics journal, I had to change all my aligns to eqnarrays for the file to compile.
But unless you are forced to, I generally recommend the align environment. Here's a good write-up of what the differences are.
If you must use the eqnarray environment, there's a package called eqnarray (available here) that at least removes the excessive space around the middle column. Compare:
\documentclass{article} \usepackage{eqnarray,amsmath} \begin{document} \begin{eqnarray*} A&=&B,\\ C&=&D,\\ E&=&F \end{eqnarray*} \begin{align*} A&=B,\\ C&=D,\\ E&=F \end{align*} \end{document} The package is for LaTeX 2.09, but it might still work.
I notice that, in my example, the eqnarray* and align* are centered very slightly differently (the eqnarray* is less than 1 point to the left of the align*). I'm not sure why. If you use {B,} and {D,} instead of B, and D, in the eqnarray*, the two displays come out centered exactly the same, so I assume that the ending punctuation symbols are causing the problem. (The default eqnarray* appears to have the same issue.)
