Since 1983, this seat has been held at Westminster by Ron Davies, who was Welsh Secretary from 1997 and who led the successful devolution campaign of that year.
Mr Davies guided the devolution legislation through the Commons, but then resigned from the cabinet in late 1998 after his much-publicised "moment of madness" on Clapham Common.
Labour's grip here was famously threatened in the 1968 by-election, when Plaid Cymru's Phil Williams came within 2,000 votes of taking the seat. However, nationalist support has since waned.
Nevertheless, in the 1999 elections, Plaid Cymru came within 3,000 votes of Ron Davies for the seat at the Assembly, and have overtaken Labour, in terms of councillors within the constituency.
The seat, which comprises the town of Caerphilly and much of the Rhymney Valley to the north, has traditionally been dependent on the coal industry, and has suffered unemployment problems in the wake of the rundown of coal.
To a large extent, manufacturing has replaced heavy industry. Many residents also commute to jobs in nearby Cardiff.