Generally, the "m"-dash (= Gedankenstrich) is printed without any spaces around it, but this is not an invariable rule; numerous publications prefer to put a space at each end of the dash. The _New York Review of Books_ (not = the _New York Times Book Review_), for example, does not place spaces between the "m"-dash and the surrounding words, whereas the _New York Times_ uses a slightly shorter dash (I believe it is called an "n"-dash) and does place spaces around it. When one is typing (saw, an e-mail on telnet), the usual practice it to symbolize the "m"-dash with two hyphens: "--" As for the spaces, it really is up to you (unless you are given a style sheet that says otherwise), as long as you are consistent. Using it without spaces does however seem to be more common, with spaces is perhaps more elevated-looking.

PS That's "em"-dash and "en"-dash...

an m-dash is --- (Gedankenstrich), an n-dash is -- (Bindestrich), and a minus is - . See LaTeX commands. This is also why it is a minus in the domain names and not a Bindestrich.

@Frank: not entirely true: a Bindestrich (hyphen) is the same as the minus sign, and definitely very different from the en-dash!.
The German Gedankenstrich is an en-dash (written "en") [alt + 0150] between spaces, whereas the Americans use the em-dash [alt + 0151] without spaces: The British tend to put spaces in, and I'd say that nowadays the en-dash is preferred over the em-dash.

Im Prinzip auf Englisch gibt es Trennstriche und Bindestriche, deren Benutzung von den Namen her ziemlich eindeutig sein sollte. Bindestriche haben keinen Zwischenraum und wird verwendet, um Wörter, die zusammen benutzt werden, zu ver"binden". Beispiele sind mehrere Wörter, die als einziges Eigenschaftswort verwendet werden z.B. up-to-date, make-up, un-American. Gedankenstriche (dash) ersetzen im Prinzip andere Zeichen wie Beistrich, Strichpunkt, Doppelpunkt. Gedankenstriche ohne Zwischenraum scheint mir verwirrend, weil man teilweise nicht weis, welches gemeint wird.

@Kevin — It's not the lack of spaces that is confusing, but the length of the dash used. Agree entirely that using a short hyphen/minus sign without spaces is senseless. But Gedankenstriche /dashes are always longer (em-dash is the width of an M, en-dash the width of an N in the same font), and so cannot be confused with a hyphen.

I must admit I am rather at a loss how the length is a problem for normal users. Are we simply talking here about typographical theory, because grammatically there is no definition of whether one or the other is longer? Maybe I misunderstood, but I don't believe that this is a big issue for normal users. Interesting what you find out though :-).

My view is that if one uses the hyphen [-] to represent an en-dash [– Alt+0150] or an em-dash [— Alt+0151], as so many do, *that* is verwirrend, because the function is unclear at first glance. And this lack of clarity means the flow is interrupted as you have to think twice as to whether or not it isn't actually linking two words.
As a tr@nslator (who also works with layouting German texts), I am constantly having to correct the wrong use of hyphens - as here - so that at least my translation is correct! And I think everyone who taskes a pride in correct punctuation should know the difference, because
"grammatically" the difference is also distinct, just like using a French accent [Ž] instead of an apostrophe ['] is wrong and look silly!

Danke an alle, die mitgeschrieben haben - es gibt also beide Varianten in der heutigen englischen und amerikanischen Typographie, das wollte ich wissen :-)

Frank, im Deutschen gibt es nebenbei vier verschiedene Arten von Strichen. Der Bindestrich (-), Abteilungsstrich (in der Antiqua gleich ausschauend wie Bindestrich), der Streckenstrich, der Strich und der Gedankenstrich(–). Diese Eigenheit der Typographie ist nicht einmal mit Unicode darstellbar, sondern nur mit Spezialprogrammen. Der englische Gedankenstrich(—)sollte im Deutschen nicht vorkommen...

Ghol, mein Favorit ist nach wie vor steht's, gesehen in einem Restaurant am Bodensee, kurz nachdem ich auf der Mainau Him*m*beeren gegessen habe. Ein falsches Apostroph, das falsch als Gravis geschrieben wurde und ein falsches stummes h - eigentlich fast eine Kunst ein so einfaches Wort wie stets so zu verunstalten ;-)

Doch wehe, neues Ungemach droht - unser Apostroph ist auch kein Apostroph, sondern ein Zollzeichen...