eqnarray vs align

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 \nonumber

Use 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.

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 &= z
• eqnarray 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)}

Besides the better spacing and the less 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}