Citizens in the 28 European Union nations will go to the polls this week in an atmosphere of uncertainty — with the specter of Brexit looming over the process and a growing nationalist, euroskeptic movement drawing voter support — to cast ballots for the bloc’s only directly elected body: the European Parliament.

The European Union is complex by design, a fact that can perplex voters and vote-watchers alike, often resulting in a low turnout.

But this time is different. Nationalists have gained ground across Europe, and as they head into the elections with a newly united front, the vote is being seen as the latest test of their influence. Polls suggest that populist parties could be positioned to make big gains.

Here’s a guide to the European Parliament elections, a notoriously confusing system made even more so by changing dynamics within the bloc.