The following is just a very short summary of social security in Belgium. The Ministry has published a good and more complete explanation : "Everything you have always wanted to know about social security"

The social security cost for employees is quite high in Belgium. 

The employer must withhold 13.07% of the gross salary, in respect of the employee's social security contributions.  On top of the gross salary, the employer also has to pay his own social security contri­bu­tions calculated at about 35% of that salary.

There are quite a number of specific social security regimes for specific types of trade. E.g. sportsmen can have an annual cap on the level of their social security contributions. 

Employees who are temporarily seconded to Belgium can continue to pay social security abroad.

Given the level of the social security contributions, some employees prefer to work as a self-employed or freelancer.

Employees who are temporarily seconded to Belgium can continue to pay social security contributions in the country where they have been working before if they were liable to social security (national insurance) as an employee there. 

This regime is governed by Regulation(EC) 883/2004 since 2010 (the old regime can be found in Regulation (EEC) 1408/71) ; it covers employees who are nationals of one of the 27 EU Member States, as well as nationals of Iceland, Liechtenstein, Norway and Switzerland.  For a general overview of the European legislation, check the Europa website.

An employee can be posted to another EU Member State for a maximum of 24 months and continue to pay social security in his home country ; his employer will give him a A1 Portal Document (the new name for the ‘E101’ certificate) that shows that exempt the employer and the employee from paying social Insurance in the country where they are posted and authorises social security to be paid to the country where he was posted from.

The employee will also receive a form E106 which gives him the same entitlement to the social security benefits in Belgium, via a Belgian mutual health fund (see hereinafter). For a short secondment, an employee should apply for a European Health Insurance Card. 

Belgium has concluded social security agreements with 26 countries outside the European Economic AreaAlbaniaAlgeriaArgentinaAustralia,Bosnia-HerzegovinaBrasilCanada (there is a separate convention for Quebec), ChileDR CongoIndiaIsraelJapanKosovoMacedoniaMoldovaMontenegroMorocco, the PhilippinesSan MarinoSerbiaSouth KoreaTunisiaTurkeyUruguay and the United States of America. Under those agreements social security contributions can be paid in the employee's country of origin.