12 April 1971: Beginning of construction of Line 1
15 August 1974: Line 1 Seoul Railway Station - Cheongnyangni (9.54 km) & Metropolitan Subway Gyeongin Line (Guro-Incheon), Gyeongsu (Seoul Station-Suwon), Cheongnyangni-Seongbuk Line
09 March 1978: Beginning of construction of Line 2
09 Dec. 1978: Metropolitan Subway Gyeongwon Line (Yongsan-Hyukong)
29 Feb. 1980: Beginning of construction of Lines 3 & 4
31 Oct. 1980: Line 2 Sinseol-dong - Sports Complex (14.3 km)
23 Dec. 1982: Line 2 Sports Complex - Seoul National Univ. of Education (5.5 km)
16 Sept. 1983 Line 2 Seongsu-Euljiro (8 km)
17 Dec. 1983: Line 2 Seoul National Univ. of Education - Seoul National University (6.7 km)
22 May 1984 Line 2 City Hall - Seoul National University (19.2 km)
20 April 1985: Line 4 Sanggye - Hansung University (11.8 km)
12 July 1985: Line 3 Gupabal-Dongnimmun (8 km)
18 Oct. 1985: Line 3 Dongnimmun - Yangjae (16.2 km), Line 4 Hansung University - Sadang (16.4 km)
02 Sept. 1986: Seongbuk-Uijeongbu Line (now operated with Line 1)
27 June 1990: Beginning of the construction of Line 5
13 July 1990: Line 3 Gupabal - Jichuk (1.5 km)
22 May 1992: Line 2 Sindorim - Yangcheon-gu Office (2.7 km)
21 April 1993: Line 4 Sanggye-Danggogae (1.2 km)
30 Oct. 1993: Line 3 Yangjae-Suseo (7.5 km)
01 April 1994: Line 4 Sadang - Namtaeryeong (2.2 km) & Urban Subway Gwacheon Line (Geumjeong-Namtaeryeong, 14.1km)
01 Sept. 1994: Urban Subway Bundang Line (Suseo-Ori 18.5 km)
15 Nov.1995: Line 5 Gangdong Wangsimni-Sangil-dong (14.5 km)
30 Jan. 1996: Urban Subway llsan Line (Jichuk-Daehwa, 19.2 km, now operated with Line 3)
29 Feb. 1996: Line 2 Sinjeong branch line Yangcheon-gu Office-Sinjeong 4way Street (1.9 km)
20 March 1996: Line 2 Sinjeong branch line Sinjeong 4way street - Kkachisan (1.4 km) & Line 5 Gangseo branch line Banghwa - Kkachisan (8.8 km)
30 March 1996: Line 5 Geoyeo zone Gangdong - Macheon (18.5 km)
12 Aug. 1996: Line 5 Youngdungpo zone Kkachisan-Yeouido (7.8 km)
11 Oct. 1996: Line 7 Gagnbuk zone Jangam - Konkuk University (19 km)
23 Nov. 1996: Line 8 Jamsil - Moran (13.1 km)
30 Dec. 1996: Line 5 Yeouido - Wangsimni (14.1 km)
03 Dec. 1996 - 22 Nov. 1999: Reconstruction of the Line 2 Dangsan Railway Bridge
02 July 1999: Line 8 Amsa - Jamsil (4.6 km)
29 Feb. 2000: Line 7 Onsu - Sinpung (9.2 km)
01 Aug. 2000: Line 7 Konkuk University - Sinpung (17 km)
07 Aug. 2000: Line 6 Bonghwasan - Sangwolgok (4 km)
15 Dec. 2000: Line 6 Sangwolgok - Yeonsinnae (27km) except stations between Itaewon and Yaksu
09 March 2001: Line 6 remaining stations opened (Itaewon - Yaksu)
03 Sept. 2003: Bundang Line Suseo - Seonneung (6.6 km)
01 July 2009: Gyeongui Line integrated into Subway system
24 July 2009: Line 9 Gaehwa - Sinnonhyeon (25.5 km)
18 Feb 2010: Line 3 Suseo - Ogeum (3 km)
21 Dec 2010: Gyeongchun Line Mangu - Chuncheon suburban railway line upgraded and integrated into Subway system
28 Oct 2011: Bundang Line Bojeong (new permanent station) - Giheung
28 Dec 2011: Sin Bundang Line (DX) Gangnam - Jeongja
30 June 2012: Suin Line Oido - Songdo
27 Oct 2012: Line 7 Onsu - Bupyeong-gu Office (10.2 km)
01 Dec 2012: Bundang Line Giheung - Mangpo
30 Nov 2013: Bundang Line Mangpo - Suwon
24 May 2014: Line 9 Magongnaru station added
21 June 2014: AREX Cheongna Int'l City station added
31 Oct 2015: Gyeongui–Jungang Line Yadang station added
For later openings see each individual line!