This is, surprisingly, a grammatically correct sentence.

It was conceived by William J. Rapaport, professor at the University of Buffalo, while an undergrad at Indiana University, and featured in Steven Pinker's The Language Instinct.

Buffalo buffalo (bison in Buffalo) Buffalo buffalo buffalo (who other Buffalo bison bully) buffalo (will bully) Buffalo buffalo (bison in Buffalo).

However, to make this clear, punctuation should be included:

Buffalo buffalo, Buffalo buffalo buffalo, buffalo Buffalo buffalo.

Um... I think.

Rappaport explains it this way:

Then, of course, you can make it more interesting by considering the buffalo in the Buffalo zoo, the Buffalo buffalo. And their peculiar way of buffaloing other Buffalo buffalo, so peculiar that, like the Tennessee waltz which you do by Tennessee waltzing, they Buffalo buffalo those other Buffalo buffalo:

Buffalo buffalo Buffalo buffalo Buffalo buffalo Buffalo buffalo Buffalo buffalo.

TO see the original:

WaldemarExkul noted to me:

Moreover, *any* number of consecutive instances of the word buffalo is a grammatical English sentence. ("Buffalo!" - 'Go out and bully (people/things)!' - "Buffalo buffalo!" - 'Bully bison!'; "Buffalo Buffalo buffalo!" - 'Bully bison from the city of Buffalo!'; "Buffalo buffalo buffalo buffalo." - 'Bison from Buffalo bully bison.'; and so on and on...)