The Tal Shiar was the elite intelligence agency of the Romulan Star Empire. It was a highly-respected and feared force in the Alpha and Beta Quadrants. Its purpose was guarding the security of the Empire, both from the Romulans' interstellar enemiesmost notably the United Federation of Planets and the Klingon Empireas well as from traitors within the Romulan population itself. The covert, often invisible presence of the Tal Shiar kept the general populace in a constant state of paranoia. Dissent and dissatisfaction with the status quo were met with severe punishment and often dissidents have been known to "disappear".
It was common practice for the Chairman of the Tal Shiar to be a member of the Continuing Committee. This was initially not the case for Chairman Koval, who was kept off the Committee, as he had often displayed anti-Federation sentiment and was opposed to the Federation Alliance against the Dominion.
The Tal Shiar reports only to the highest levels of the Romulan government, and even so they undertake many missions without the direct knowledge of the Romulan Senate, conducting operations with virtual impunity throughout the Empire. Their authority supersedes that of most Romulan military commanders, resulting in a mutual distrust and hatred between the two organizations. The Tal Shiar operates their own fleet, often to their own aims.
In 2369, the Romulan underground movement forced Counselor Deanna Troi of the USS Enterprise-D to pose as Major Rakal of the Tal Shiar in order to facilitate the defection of Vice-Proconsul M'ret and his aids to the Federation. The real Major Rakal was killed prior to the operation.
In 2371, the Tal Shiar formed a secret coalition with the Cardassian Obsidian Order, to carry out a preemptive strike against the Founders' homeworld in the Dominion. As part of this operation, they carried out the assassinations of several former Obsidian Order operatives, including (unsuccessfully) Elim Garak. However, the Tal Shiar itself had been infiltrated by the Founders, and their combined fleet of twenty ships was ambushed and annihilated at the Battle of the Omarion Nebula.
Unlike the Obsidian Order, the Tal Shiar was able to be rebuilt after the attack on the Founder homeworld. In 2374, operatives of the Romulan Military hijacked the prototype starship USS Prometheus and attempted to bring it to the Empire. They were supposed to bring it to the Military but received orders during the operation to deliver it to the Tal Shiar's fleet instead.
In late 2374, the Tal Shiar investigated the destruction of Senator Vreenak's shuttle. Their preliminary report pointed towards Dominion sabotage.
The chairman of the Tal Shiar during the Dominion War, Koval, was in fact a Federation collaborator who was planted in preparation for feared hostilities following the war. Following covert maneuvering orchestrated by Section 31, Koval became a member of the Continuing Committee from which he had been previously excluded.