A couple of people have suggested eqnarray which will work, but a better approach is the align environment in the amsmath package.

egin{align*}
something \
something else
end{align*}

If you want the two parts aligned in some way, use an & as described in the amsmath documentation.

Instead of using the TeX-style $$ commands, consider using the align* or gather* environments. Inside those, you can use the line break command \.

(You will need to load the amsmath package to use those; but if you re doing any math at all, you should be loading that package regardless.)

11 years later...

An example of breaking text in multiple lines is having a cell with multiple lines in an array. Instead of using a new array row, you can break the text in lines within a cell. The advantage of doing this is interline space is not dependent on the whole row height (if some cell in the row has a large height, this won t influence the text interline space):

To do this, just enclose the cell content within curly braces and use \ as a linebreak.

Content of the highlighted cell:

{ T_f (u) 	ext { is the transformed function and }
  \
  K (t,u) 	ext { is the kernel used. }}

Whole code:

$egin {array} {lll}
	ext {Transform: } T_f (u) 
    & = int limits_{t_1}^{t^2} K (t,u) f(t) dt ;; 	extrm {where:}
        & {
            T_f (u) 	ext { is the transformed function and }
            \
            K (t,u) 	ext { is the kernel used. }
          }
          \
	ext {Inverse transform: } f(u) 
    & = int limits_{u_1}^{u^2} K^{-1} (t,u) T_f(u) du
end {array}$

The way to get line breaks in display math, while using only standard LaTeX, is to use egin{array}...end{array} within the display math environment $$...$$. (Yes, I know that $$ is deprecated but I still like it.) There are many alternatives in different extensions, including AMSLaTeX s align and align* environments. My personal favorite is Didier Rémy s mathpartir package, which gives a display-math environment that is more like paragraph mode, plus a set of tools for typesetting logical inference rules and proof trees.

Try something like this:

    documentclass[varwidth,margin=3mm]{standalone}
    usepackage{amsmath}
    egin{document}
        egin{math}
                alpha(e1)=c1, alpha(e2)=c3\
                alpha(e3)=c5, alpha(e4)=c2\
                alpha(e5)=c4, alpha(e1)=c1\
        end{math}
    end{document}

The varwidth parameter is the key.
With it you can properly utilize linebreak, ewline, \, etc.
I used egin{math} / end{math}, but you should be able to use the old $ / $$ notation too.

I think a simple solution in many cases (when a math mode is present inside a paragraph) can be to just split the single line $---segment1-----|--segment2-----$ in math mode (where a split is desired at the symbol "|") to two consecutive segment each inside math mode as $---segment1-----$$--segment2-----$.