Citizens of United States of America do not require visas for travel to Hong Kong and as it is one of the visa exempt countries.
Hong Kong allows visa-free entry to over 145 countries with the validity ranging from 7 to 180 days. The purpose of the travel can be tourism or business related activities. The visitors must stay only in the Hong Kong Special Administrative Region during the stay. You can extend the validity of your stay by granting permission before coming to Hong Kong or before the entitled visa free period expires. You must have a valid passport with validity of more than 1 month. 
Congratulation! Citizens of below nationalities can avail of evisa online or visa on arrival to travel for certain propose:
1- A recent digital photograph of the applicant.
2- Scanned color copies of your passport with minimum validity of up to 6 months.
1- Hong Kong offers 180 days visa-free stay to the Citizens of United Kingdom and Macau and 90 days visa-free stay to the citizens of 85 other countries.
2- Visa free stay is also provided to the citizens of 35 countries for 14 days and 23 countries for 30 days.
3- 7 days visa-free stay is granted to the citizens of East Timor.
4- On every visit, the visitors must stay in the HKSAR.
5- You will be prohibited on taking any sort of paid employment or establishing any business during your stay in Hong Kong.
6- You can extend the validity of your stay by granting permission before coming to Hong Kong or before the entitled visa free period expires.
All Countries / Nationalities that required regular visa for Hong KongApplication for the Long Stay / employments Visas, contact to these embassy below
Hong Kong Trade Office in Washington, D.C., United States
1520, 18th Street, NW
Washington, D.C. 20036
United States
http://www.hketowashington.gov.hk
Hong Kong Trade Office in New York, United States
115 East 54th Street
New York, NY 10022
United States
Hong Kong Trade Office in San Francisco, United States
130 Montgomery Street
San Francisco, CA 94104
United States