To enter the Hong Kong Special Administrative Region (SAR) or the Macau SAR you need a valid passport. On entry German nationals receive a tourist visa which is valid for 3 months. This visa is, however, not valid for travels into Mainland China.
Starting from 16 July 2018, passengers arriving in Hong Kong who are in possession of currency and bearer negotiable instruments with a total value of over HKD120,000 will be required to make a written declaration using the Red Channel (R32 System Declaration). Details can be found on the website of Hong Kong Customs.
To enter the P.R. of China you will need a Chinese visa for which you have to apply before the journey at the Chinese Embassy or one of the Chinese Consulates General in Germany.
For further enquiries please refer to:
Visa Section of the Consular Department of the Commissioner's Office of China's Foreign Ministry in the Hong Kong SAR
A lot of German tourists and businessmen travel via Hong Kong to other destinations in Southeast Asia. Please be aware that for most of these countries your passport has to be valid for at least another 6 months after your journey (e.g., Philippines, Thailand, Malaysia, Indonesia, Taiwan). Please refer to the immigration authorities of the respective countries for details.
The Federal Foreign Office's website provides current information on the security situation in Hong Kong, Macau, Taiwan and Mainland China. The service is available in German language.