Line 1 is the oldest of the three lines of the Athens Metro, running from Kifissia to Piraeus. The Athens-Piraeus Railway Company first opened the line, between Piraeus and Thiseio, on 27 February 1869. On 4 February 1885 Lavrion Square-Strofyli railway line opened between Attiki Square and Kifissia. These railway lines gradually merged and converted to a rapid-transit system. It was merged into the Athens Metro network upon the opening of Lines 2 and 3 on 28 January 2000.

The Athens Metro is a rapid-transit system in Greece which serves the Athens urban area. Line 1 opened as a single-track conventional steam railway in 1869 and was electrified in 1904. Beginning in 1991, Elliniko Metro S.A. constructed and extended Lines 2 and 3. It has significantly changed Athens by providing a much-needed solution to the city's traffic and air pollution problem, as well as revitalising many of the areas it serves. Extensions of existing lines are under development or tender, like the Line 2 extension to Ilion where tender started in 2023, as well as a new Line 4, whose central section began construction in October 2021. The Athens Metro is actively connected with the other means of public transport, such as buses, trolleys, the Athens Tram and the Athens Suburban Railway. The Athens Metro is hailed for its modernity, and many of its stations feature works of art, exhibitions and displays of the archaeological remains found during its construction. Photography and video-taking is permitted across the whole network and street photographers often work in Athens Metro. This will be the only metro system in Greece, until the Thessaloniki Metro begins operation in 2024.