More than 9,500 seats in 279 councils across England are being contested, along with a by-election in Leicester South and several mayoral elections.
There are national elections to the Scottish Parliament, and Wales and Northern Ireland Assemblies.
There will also be a UK-wide referendum on whether to adopt the alternative vote (AV) for Westminster elections.
The battle between Yes and No to AV supporters has become increasingly bitter in recent days, with reports of a confrontation between Lib Dem Energy Secretary Chris Huhne and the prime minister at a cabinet meeting on Tuesday.
The Conservatives want to retain the existing first-past-the-post voting system, while their coalition colleagues, the Lib Dems, want AV. Labour are split on the issue, but leader Ed Miliband is in the Yes camp.Spending cuts
In the English local elections, the Conservatives are defending about 5,000 seats, which were last contested in 2007. Labour and the Lib Dems are fighting to hold onto about 1,600 and 1,800 seats respectively.
Among the 36 metropolitan councils electing a third of their seats are Birmingham, Manchester, Liverpool and Sheffield. In addition, elections will be held in 49 unitary authorities and 194 district councils.Continue reading the main story
At the moment MPs are elected by the first-past-the-post system, where the candidate getting the most votes in a constituency is elected.
On 5 May all registered UK voters will be able to vote Yes or No on whether to change the way MPs are elected to the alternative vote system.
Under the alternative vote system, voters rank candidates in their constituency in order of preference.
Anyone getting more than 50% of first-preference votes is elected.
If no-one gets 50% of votes, the candidate with the fewest votes is eliminated and their backers' second choices allocated to those remaining.
This process continues until one candidate has at least 50% of all votes in that round.Much of the election campaign has focused on the issue of government spending cuts, with claims and counter-claims being made about their impact on council services.
Labour leader Ed Miliband, who will visit Gravesend, in Kent, on Wednesday, has urged voters to "send a message" to the government that they feel "utterly betrayed" by their policies.
The Conservatives have said that while local authorities are facing difficult decisions, many have demonstrated that they can do "more for less" by cutting waste and sharing services.
The Lib Dems, meanwhile, say councils they control have not cut vital services like libraries and children's centres - unlike Labour and Conservative authorities.
Their leader Nick Clegg will visit Leicester South on Wednesday where a by-election is being held after the Labour MP, Peter Soulsby, stood down to take part in the first mayoral contest in the city.
Other parties, including the UK Independence Party, the Green Party and the British National Party, are also hoping to make electoral gains.
The Greens currently have 60 councillors, UKIP has 19 councillors and the BNP has 21 - but is defending only 12.TV debates
In Scotland, 129 MSPs will be elected to the Holyrood parliament - 73 through the constituency-based first-past-the-post system and 56 through a separate party list system in eight electoral regions.
At the last vote in 2007, the SNP emerged as the largest party and formed a minority government, but constituency boundaries have been significantly redrawn since then and one in six voters are now in a different seat.
The four main party leaders, Iain Gray, Alex Salmond, Annabel Goldie and Tavish Scott, went head-to-head on Tuesday in a televised debate.
In Wales, 44 Assembly Members will be elected - again using a mixture of first-past-the-post voting linked to constituencies and AMs elected from a party list.
This is the first poll in Wales since 2007, after which Labour formed a coalition with Plaid Cymru.
In a televised debate in Cardiff on Monday, the leaders of Labour, Plaid Cymru, the Conservatives and the Lib Dems argued over public sector cuts and standards in schools.
In Northern Ireland, 108 seats at the Stormont Assembly are up for grabs.
Currently, the Democratic Unionists are the largest party in the power-sharing body with eight more seats than Sinn Fein.
Northern Ireland voters will also go the polls to elect 582 councillors in the first local elections for six years.
As well as Leicester, polls will also be held to elect mayors in Middlesbrough, Mansfield, Bedford and Torbay.
According to a ComRes poll of 1,033 people for the Independent, support for the No to AV campaign has grown in the past week.
The survey found that 66% of respondents were opposed to AV, while 34% supported it - compared with 60% and 40% respectively a week ago.
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