Around 45 million registered voters will have the chance to cast their ballots in a slew of elections across the UK on "Super Thursday".
To add to the complexity, four different voting systems will be in use.
What are people voting on and where?
There are 2,743 seats up for grabs on 124 councils using the first-past-the-post system.
As usual, Sunderland will be among the first to declare, from around 11pm on Thursday. Final results are not expected until about 5pm on Sunday.
* Police and Crime Commissioners (PCCs) for each of 36 force areas are elected using the Supplementary Vote system.
Votes for most other polls will be counted first, with PCC counts taking place in the day on Friday.
The PCC election due in Greater Manchester has been cancelled as the commissioner's powers are due to be transferred to a new directly-elected mayor next year. London similarly has no PCC as powers overseeing the Met fall to the mayor.
There are mayoral elections at Bristol, Liverpool and Salford.
The full complement of 129 Members of the Scottish Parliament (MSPs) are being elected for a five-year term.
Of those, 73 are elected by first-past-the-post (FPTP) for each of the constituencies and another 56 are elected from party lists according to the strength of each party in a separate vote - adjusted against the number of FPTP seats won - to represent the eight electoral regions.
First results are expected around 2am on Friday with the final ones expected around 8am, but they could be much later if there are delays at counts.
The full complement of 60 Assembly Members (AMs) are being elected for a five-year term.
Of those, 40 are elected by first-past-the-post for each of the constituencies and another 20 are elected from party lists according to the strength of each party in a separate vote, adjusted against the number of FPTP seats won, to represent the five electoral regions.
First results are expected around 1am on Friday with the final ones expected around 7am, but they could be much later if there are delays at counts.
Also, Police and Crime Commissioners (PPCs) for each of four force areas - Gwent, Dyfed-Powys, North Wales, South Wales - are being elected using the Supplementary Vote system.
Votes for the assembly polls will be counted first, with PCC counts taking place for the four police areas on Sunday.
The full complement of 108 Members of the Legislative Assembly are being elected for a five-year term.
There are six representing each of the 18 constituencies, elected using the Single Transferable Vote system.
Counting will start during daytime on Friday with the first declarations expected at about 1pm and the final results expected late on Saturday.
The mayor is elected for a four-year term under the Supplementary Vote system.
The result is expected late on Friday evening.
With the Greater London Assembly, 14 members are elected by first-past-the-post to represent individual constituencies and another 11 from party lists.
Results are expected from around 3pm on Friday.
There are two Parliamentary by-elections with MPs to be elected by first-past-the-post to represent Ogmore and Sheffield Brightside & Hillsborough (following the resignation of Huw Irranca-Davies to fight for a Welsh Assembly seat and the death of Harry Harpham).