Will there be a solo campaign?

Yes. Project leader Jason Jones has stated that the solo campaign will be an important part of Halo, giving it much of its personality and fleshing out its story. Plus, it substantially increases Halo's audience.

In addition, this solo campaign can be played through by two players working cooperatively. Currently this is only possible for two players only, using the same Xbox console using that hardware's split screen mode.

It is not known what cooperative modes, if any, will be available for the Windows and Macintosh version of Halo or what limitations it would have.