The Hang Seng Index (abbreviated: HSI, Chinese: 恒生指數) is a freefloat-adjusted market capitalization-weighted stock market index in Hong Kong. It is used to record and monitor daily changes of the largest companies of the Hong Kong stock market and is the main indicator of the overall market performance in Hong Kong. These 48 constituent companies represent about 60% of capitalisation of the Hong Kong Stock Exchange.^{[citation needed]}

Since 7 March 2011, the HKEX extended their trading hours. In the first stage, (opening value will be at 09:20) 09.30-12.00 and 13.30-16.00. In the second stage, from 5 March 2012, the afternoon trade will change to 13.00-16.00, that's mark with the mainland trading hours.

HSI was started on November 24, 1969, and is currently compiled and maintained by Hang Seng Indexes Company Limited, which is a wholly owned subsidiary of Hang Seng Bank, one of the largest banks registered and listed in Hong Kong in terms of market capitalisation. It is responsible for compiling, publishing and managing the Hang Seng Index and a range of other stock indexes, such as Hang Seng China Enterprises Index, Hang Seng China AH Index Series, Hang Seng China H-Financials Index, Hang Seng Composite Index Series, Hang Seng China A Industry Top Index, Hang Seng Corporate Sustainability Index Series and Hang Seng Total Return Index Series. Hang Seng in turn, despite being a public company, is held in majority by British financial firm HSBC.

Hang Seng Bank Limited (Chinese: 恒生銀行有限公司) (SEHK: 0011, OTC Pink: HSNGY) is the second largest bank in Hong Kong. It is a listed company but it is majority owned (62.14% equity interest) by the HSBC Group via The Hongkong and Shanghai Banking Corporation. Hang Seng is also one of the constituent shares of the Hang Seng Index. The current vice-chairman and CEO is Margaret Leung Ko May Yee. Former CEO Vincent Cheng has been appointed to be the Chairman of the Hongkong and Shanghai Banking Corporation, and took up the post on May 25, 2005. Its headquarters are in Central.