The most contentious vote takes place in Scotland, where Labour faces a substantial challenge from the Scottish National party (SNP).
In total 129 members of the Scottish parliament will be elected following voting in 73 constituencies, in addition to 1,222 council seats across 32 councils.
Late polls suggest Labour has narrowed the gap behind Alex Salmond's nationalist party but the SNP are still expected to come out on top – just.
Elsewhere, the third elections to Wales' National Assembly take place across the province's 40 constituencies.
Here Labour is also expected to fare badly. Welsh party leader Rhodrig Morgan has called on voters to reject a "pig in a poke" coalition and hand Labour another majority but polls suggest the nationalist Plaid Cymru party and the Conservatives are set to do well.
England also has its share of voting, with 312 councils holding polls.
But the hopscotch nature of English local government means only district councils and some metropolitan and unitary councils are holding elections. Some councils have all their seats up for grabs while others are offering just a third of the seats.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.