In the run-up to the general election, we will hear a lot more about how the parties are doing in the opinion polls and who the next government might be.
But how do the polls work, whose opinions are asked and how valuable are the results?
What is an opinion poll?
Opinion polls provide a snapshot of people's views on a particular issue. They are widely used when elections are approaching.
People may be asked things like which party they would vote for, their views on party leaders and which issues most concern them.
Opinion polls can be useful to see how different parties are doing, but they may not reflect the make-up of the next Parliament.
The UK's first-past-the-post voting system means that the local candidate in each constituency who receives the most votes becomes an MP. And the party with the most winning candidates nationally normally forms a government.
So, a party that is predicted to have 10% more of the vote than its main rival, for example, will not always end up with 10% more MPs.