Italy's current complex and much-debated electoral system represents a compromise between a majority system and proportional representation.
Three-quarters of the seats in both houses are determined by a first-past-the-post vote in single-seat constituencies, while the remaining 25 per cent are decided by a proportional voting system.
The 25 per cent proportional vote theoretically safeguards the interests of minor parties since votes are subtracted from the party or coalition whose candidate won the direct-majority vote. Minor parties must poll at least 4 per cent of the vote in order to win a proportional seat.
Controversially, both the major centre-left and centre-right coalitions have found a way round this handicap by the use of "decoy lists". This means they run their candidates under a false or "decoy" party name affiliated to the coalition. If the candidate wins, votes are subtracted not from his coalition (centre-left or centre-right) but rather from his "decoy" party.
The current electoral system is the outcome of reforms introduced in the early 1990s. These followed a series of referendums in which the electorate voted overwhelmingly in favour of change. Before 1993 Italy had a pure PR system. Voters elected parties and candidates within the parties they had chosen. Seats were then divided up according to the proportion of the total vote each party received. Parties would allocate seats to their candidates, based on the number of votes each received in his or her district.
Reuters adds: According to the Interior Ministry, 173 political parties and individual candidates are standing for election.
The biggest party, the Democrats of the Left, led by Mr Walter Veltroni, was born when the Italian Communist Party, once the biggest communist party in the West, renounced Marxism, renamed itself and changed its symbols.
The conservative Forza Italia party was founded by Mr Silvio Berlusconi in January 1994, four months before winning a general election. Forza Italia won the biggest share of the vote in regional elections in April 2000.
The right-wing National Alliance was officially founded in 1995 out of the neo-fascist Movimento Sociale Italiano after reforms leading the party to reject racism, anti-Semitism and totalitarianism.
The Northern League, an anti-immigrant party, was founded by Mr Umberto Bossi and first advocated the independence of the rich north. It now wants devolution.
Other parties include Communist Refoundation, the rump which remained after the main Italian Communist Party renamed itself, two small Christian Democrat formations, the Greens and the small Democrats party, founded by European Commission president, Mr Romano Prodi, and including in its ranks the centre-left's candidate for prime minister, Mr Francesco Rutelli.