Treaty of Algeron
The Treaty of Algeron is an agreement between the United Federation of Planets and the Romulan Star Empire that somehow "keeps the peace" between the two powers, who presumably negotiated it following the events of "The Enterprise Incident" (TOS).
Under the terms of the treaty, the Federation may not develop or deploy cloaking technology, resulting in a significant tactical disadvantage for Federation starships. The Treaty also reinforces the Romulan-Federation Netural Zone, declaring any intrusion by either power into the Zone a act of war.
In 2371 (in Star Trek year terms), a secret amendment was added to the Treaty, allowing for Federation use of at least one cloaking device, but under Romulan supervision. It's use was to be restricted to the Gamma Quadrant. In turn, the Romulans gained extensive information on the exploration voyages of the USS Defiant within the Gamma Quadrant, thus producing another tactical disadvantage for Federation ships.
References at http://memory-alpha.org/wiki/Treaty_of_Algeron.