Ttyjukl

Wikipedia's page on Voter Turnout states the following about obligatory attendance in Belgium:

In both Belgium, which has obligatory attendance, and Malta, which does not, participation reaches about 95%. In Belgium there is obligatory attendance which is often misinterpreted as compulsory voting.

If I understand correctly, eligible voters need to show up at the polling station but they do not need to cast a vote (or they can cast an empty ballot?).

How does Belgium enforce those who don't comply? That is, how does Belgium deal with (or punish) those who fail to show up?

The King's public prosecutor puts together a list of people who didn't vote and they are summoned to court. Depending on how many times they've skipped voting, they will either receive a small fine or be ineligible to vote for 10 years and have issues if working for a public authority. This can be contested, and doesn't appear to be enforced these days.

Article 209 and 210 of the Belgian electoral code covers the punishment guidelines:

Art. 209. Within 8 days of the proclamation of the representatives, the public prosecutor of the King puts together a list of the voters who abstained from voting without valuable excuses.

These voters are summoned up by simple reprimand before the criminal court which renders a judgment without appeal, the prosecuting body being heard.

If they are Belgian citizens living abroad and registered in the consular register of the population, the prosecutor of the King hands out the list to the Federal Public Service Foreign Affairs, Foreign Commerce and Development Cooperation. It ensures that the mention of the registration municipality of the voters concerned is eliminated from the consular registry of the population by the diplomatic posts or consulates.

Art. 210. A first arbitrary absence is penalized with a reproval or fine of five to ten euros depending on the circumstances.

In case of recurrence the fine shall be of ten to twenty-five euros.

An additional prison sentence shall not be pronounced.

If the arbitrary absence occurs at least four times over a time period of fifteen years, the voter shall be deleted from the voters’ list for ten years regardless of his/her criminal record. During this time he cannot receive any nomination nor promotion nor distinction from any public authority.

In the cases provided by the present article the suspension of sentence on probation cannot be ordered.

The conviction can be contested over a period of six month after the notification of the judgment. The appeal can be made by a brief statement at the municipal administration free of charge.

It's not clear if the Belgian government's official policy is to enforce this or not, though at least one source claims that no fines have happened since 2003 simply because the public prosecutor hasn't filed any non-voting cases. If that is the case, then it seems like the enforcement method is the potential of small fines if enough people begin shirking their voting duty.

This paper by Elżbieta Kużelewska, published in the journal Białostockie Studia Prawnicze (Bialystok Legal Studies) goes into the history and practice of this policy:

Before the elections each registered voter receives a polling card together with a summons (lettre do convocation). Each person registered on the electoral roll in each local authority has the duty to participate in elections

...

Non-participation in elections carries the risk of sanctions as specified under Chapter VI of the Belgian Electoral Code w Art. 207-210

...

Voters who are unable to participate in voting present the magistrate with reasons for absence together with required proof. Within eight days of the results of the elections being declared, the crown prosecution drafted a list of voters who did not participate without a leave of absence. Those voters face criminal proceedings in court. In Belgium the fine for failing to turn up on elections day for the first time is €5-€10 (the magistrate may increase it to €30-€60), for the second time €10-€25 (and even €60-€150) 43. Voters who do not participate in elections and whose absence was not excused pays a fine without a right of appeal. If a voter fails to attend at least four times in 15 years they are struck off the electoral register for the period of 10 years and in that time they may receive no nomination or hold a public post.

However sanctions beyond the fine are rarely enforced:

It is worth highlighting that statutory sanctions for failing to meet the electoral duty tend to stay on paper, with the exception of the fine. Since 2003 there have been no convictions for failing to participate in elections due to the fact that, because of the number of pressing cases, the crown prosecution has not filed any cases

Citation:

Kużelewska, Elżbieta. (2016). Compulsory Voting in Belgium. A Few Remarks on Mandatory Voting. Białostockie Studia Prawnicze. 20/A en. 37-51. 10.15290/bsp.2016.20A.en.03.

https://www.researchgate.net/publication/328169667_Compulsory_Voting_in_Belgium_A_Few_Remarks_on_Mandatory_Voting

You can download the cited sections of the Belgian Electoral Code from the Council of Europe's VOTA database here: https://vota.te.gob.mx/page/countrie/14

The relevant section is Title VI: On mandatory voting and sanctions