A total of 4,851 council seats are up for grabs in 88 councils - all of those in Scotland and Wales, plus 34 in England, including 27 counties.
Six new "metro mayors" will be elected, covering areas including Greater Manchester, the West Midlands and the West of England.
Polling stations will open at 07:00 and close at 22:00 BST.
Some English and Welsh councils will begin counting ballots as soon as polls close - while others will start counting on Friday morning, with results continuing all day Friday.
Scotland's 32 councils will start their counts on Friday morning, with first results expected from midday. Most mayoral results will be declared on Friday.
A cloudy start is expected across much of the UK with light rain anticipated in parts of Wales, London and the south east - while sunny spells are expected in north west England and Scotland.
In England, (34 councils, 2,370 seats) this is a year of county council elections, but there will also be polls in six unitary authorities where county councils and district councils have merged.
In Scotland polls will be held in all councils (1,227 seats) and all 22 councils in Wales (1,254 seats).
For the first time voters in Greater Manchester, the Liverpool City region, the West Midlands, Tees Valley, the West of England, and Cambridge and Peterborough will elect new "metro mayors", covering combined local authority areas.
Doncaster and North Tyneside are also voting for local authority mayors, who are elected leaders of their respective councils.
Voters are set to go to the polls for local and mayoral elections in England, Wales and Scotland.