All voters are called up to vote, with registration being automatic and voting being mandatory. However, none of the legally existing penalties or sanctions have ever been enforced.
250 seats are distributed on the basis of proportional representation, with a threshold of 3% required for entry into parliament. Blank and invalid votes, as well as votes cast for parties that fall short of the 3% threshold, are disregarded for seat allocation purposes. 50 additional seats are awarded as a majority bonus to the party that wins a plurality of votes, with coalitions in that regard not being counted as an overall party but having their votes counted separately for each party in the coalition, according to the election law. Parliamentary majority is achieved by a party or coalition of parties that command at least one half plus one (151 out of 300) of total seats.
The next general election cannot be held later than Sunday 20 October 2019. According to the Greek constitution, Members of Parliament are elected for terms of four years, with elections required within thirty days of their terms expiration. 
^Προεδρικό Διάταγμα 96/2007 [Presidential Decree 96/2007] (in Greek). Article 117, Clause 1: The elector who unjustifiably does not vote is punished with imprisonment of [at least] one month and up to one year.