ESI was created in the aftermath of the 1972 Olympics in the mad rush of countries forming teams to protect themselves from terrorism. Originally called Le Group Diane, ESI's cadre was filled with a small group of willy and dedicated operators. Although the name was changed to ESI in 1974, the name Diane is still used by the Belgian press.

ESI is used in a very agressive role, performing both anti and counter-terrorist activities. Not only do they root out terrorists (among a wide range of others) but the take active measures to prevent terrorist teams from operating in country.

As with other CT and elite units, service in ESI is voluntary. ESI hopefuls must undergo a rigorous weeding out phase design not so much to challange their physical prowess as much as their mental toughness. The selection process lasts two weeks and has an attrition rate of nearly 50%.

Those who pass the selection phase face another three months of training before they become fully qualified operators. ESI is different from many of the western units in that females are permitted to serve in the unit, although they serve in observation and undercover roles only.

The main weapon in the ESI arsenal is the ubiquitos HK MP-5 family. Remington 12-guage shotguns are also used for room clearing. Pistols include the Browning 9mm and Glock 17. Snipers are issued the Finnish Sako TRG-21 7.62 or a self-designed 7.62 police sniper rifle. Other weapons like the Smith & Wesson .38 revolver and HK model 94 submachine gun are also used.

ESI operators recieve training in the offensive and tactical use of cars during high-speed chases and hostage resuce. Their motor pool contains unmarked vans and Mercedes sedans used in undercover work as well as Range Rover 4x4s. Various motorcycles are also used. Zodiac inflatable boats are used for seaborne operations.

ESI is used for far more than CT work. In 1993 they spent 1,000 man hours in counter-terrorist duties. In the same year they spent 7,000 hours fighting narcotics trafficing and nearly 20,000 man-hours in action against organized crime. In one instance on April 30, 1990, ESI was sent in to take down two armed men who tried to rob a bank and wound up barricading themselves inside with hostages. The operators from ESI blew the door and entered in the smoke and confusion. Within seconds of the blast the men were isolated and captured, with no casualties.