The Southern Railway operated a large and diverse fleet of electric multiple units.  Most pre-war suburban units were conversions of locomotive-hauled rolling stock originally built by the Southern's constituents, the London and South Western Railway (LSWR), the London Brighton and South Coast Railway (LBSCR) and the South Eastern and Chatham Railway (SECR).  As the Southern expanded the electrified network beyond the suburban area, it added newly-built main line fleets, many of which (such as the 6 Pul. unit in the picture) offered high standards of comfort and amenity.

The history of each of the Southern Railway's fleets of electric multiple units is described in the pages below:

• The LBSCR a.c. fleets
• LSWR and SR 3-car suburban motor units and 2-car trailer sets
• 4-car suburban units (4 Sub)
• 2-car units for the South London line (2 SL)
• 2-car units for the Wimbledon-West Croydon line (2 Wim)
• 4-car units for main line semi-fast and stopping services (4 Lav)
• 2-car units for main line semi-fast and stopping services (2 Bil)
• 2-car units for main line semi-fast and stopping services (2 Hal)
• 2-car units for main line semi-fast and stopping services (2 Nol)
• 6-car express units (6 Pul. and 6 Cit)
• 6-car express units (6 Pan)
• 5-car Pullman express units (5 Bel)
• 4-car express units (4 Cor, 4 Res, 4 Buf, etc.)
• 6-car express units (6 Cor) 

The descriptions of the main line fleets are based on a series of articles titled "The First Generation of Southern Electric Main Line Stock" by "SCWS" that first appeared in the Southern Electric Group's magazine "Live Rail" in the late 1970s.  The pre-war suburban unit descriptions are based on an unpublished monograph by SEG Member Richard Bell.  The material relating to the post-war 4 Sub units is based on "The 4 Sub Story", by Bryan Rayner and David Brown, published by the SEG in 1983.  SEG Member John Lumley contributed a considerable amount of additional material.