Founded in 1948 by a British army officer, Major General William Cawthorne, the ISI ballooned in the 1980s when the CIA entrusted it with billions of dollars of assistance for mujahideen rebels fighting Soviet forces in Afghanistan. It is thought to have 10,000 employees, three-quarters of whom are serving army officers on secondments from other units. The remainder is a mix of civilians and retired officers.
Internally the ISI is divided into lettered sections, the most notorious of which is the S wing, which manages the relationship with Islamist militant groups. The C wing liaises with foreign intelligence services, and includes a CIA-funded counter-terrorism centre. Quite often, western spies complain, the C wing says one thing while the S does another.
Theoretically the ISI reports to
Pakistan's prime minister, Yousaf Raza Gilani. In reality it answers to the army chief, General Ashfaq Kayani. It is much more powerful than Pakistan's other spy outfits, Military Intelligence (MI) and the civilian Intelligence Bureau (IB).
Pakistan: How the ISI works | World news | The Guardian