Starting at Whampoa in Hung Hom and ending at Tiu Keng Leng in Tseung Kwan O, Sai Kung, the route has 17 stations and takes 35 minutes to complete.

[3] Opened on 1 October 1979 as the first urban railway line in Hong Kong and the first operated by the Mass Transit Railway Corporation (MTRC), the Kwun Tong line operates over much of the original section of the "Modified Initial System", from Shek Kip Mei to Kwun Tong station, which it is named after.

It was predated only by the suburban East Rail line, which at the time was in the process of being electrified and upgraded to a commuter service as part of the Kowloon–Canton Railway.

When the Tsuen Wan line started service in May 1982, it took over the section of the Modified Initial System south of Argyle (present-day Mong Kok).

The incident was blamed on a bolt in the train's suspension system which had worked itself loose, causing the weight load to be concentrated on the rear wheels of the carriage.

[6] As part of a project to reduce congestion at Quarry Bay, the Kwun Tong line was briefly extended to North Point on 27 September 2001.

The benefits would be a better transfer arrangement at Ho Man Tin and other SCL interchange stations for services to the northeastern and northwestern New Territories.

Passengers would be able to change to the North-South corridor at Hung Hom for cross-harbour services, which would terminate at Admiralty after Central South station was removed from the final plan.

[10] The 2.6-kilometre (1.6 mi) Kwun Tong line extension (abbreviated KTE; Chinese: 觀塘綫延綫; Jyutping: Gun1 Tong4 Sin3 Jin4 Sin3) from Yau Ma Tei to Whampoa via Ho Man Tin began construction on 25 July 2011[11] and opened for service on 23 October 2016.

Yau Tong and Tiu Keng Leng stations provide cross-platform interchanges in the same manner as Mong Kok and Prince Edward.