Since conventional police forces are no longer able to handle the constantly growing number of crimes on their own, private security agencies have become valuable preventive factors in such areas as industrial security, department store security, protection of money transports, and in the monitoring of security systems. According to Belgian law, the legal limitations of private police activities include clear identifications of private agents so that they cannot be confused with public police forces, limitations on bearing arms and making arrests, submission to public supervision for all activities on public terrain, and the obligation to hire and maintain only security agents who have no criminal record. Today, approximately a dozen private Belgian security agencies exist, most of which are branches of foreign or international organizations. 'Securitas', which may be considered a typical security organization, employs a staff of 1,300, runs its own security training program, maintains 200 vehicles (most of them armored), and monitors its own electronic security system 24 hours a day. While most management positions are filled by former police officers and security specialists, Securitas does not hire professionals for the field work fearing that former police officers through force of habit might overstep their legal limitations. The article's suggestions for increasing the effectiveness of private organizations include improving the collaboration between private and public forces, making the presence of private security forces more obvious (e.g., in department stores), extending their powers for arrest, and requiring better protective (i.e., bullet-proof) clothing. --in French.