For all intents and purposes, the align environment is a replacement for the eqnarray environment and all its warts. Rather than \begin{eqnarray*} x^2 + y^2 &=& 1 \\ y &=& \sqrt{1 - x^2}, \end{eqnarray*} one can type \begin{align*} x^2 + y^2 &= 1 \\ y &= \sqrt{1 - x^2}. \end{align*}