Unlike Westminster elections, you do not need to bring ID or your polling card with you to vote.
This is the second Welsh parliamentary election allowing 16 and 17-year-olds to take part.
Another change at this election is that voters will cast their ballot once, for a political party or independent candidate.
At previous Senedd elections there were two votes – one for a candidate and another one for a political party.
Elections are also taking place to the Scottish Parliament on Thursday, and local elections and mayoral contests are happening in some parts of England.