All foreigners visiting China for the Olympics or otherwise is required to obtain a visa prior to entry. Single and multiple visas are available for up to 30 days. Extensions can be granted when in the country. Check also that your passport is valid for more than six months at the time you start traveling. Many countries will not grant you a visa if your passport expires in six months time.

For visits to Hong Kong no visa is required for most western countries. However, if you exit China to go to the Hong Kong and do not have a multiple entry visa, you will need to get a new visa to enter China again.

Visas are issued through Chinese embassies and consulates around the world. Some travel agencies can also organize your visa on your behalf, usually for an additional fee. It takes at least four working days for the visa to be issued, although it can be done quicker if you pay additional fees. Get your application in early to save money and hassle, especially with the expected rush leading up to the Olympic Games.

For US citizens the visa is $100 and $80 for people on group tours.

Here is a link to Chinese embassies around the world.