The Tilonians were characterized by arches that fade from the middle of their forehead, going around their temple, and fades back away by their cheek bones. Inside the arches were a series of ridges. A common hair color among Tilonians was dark brown.
In mid-2369, the Tilonian Prime Minister was assassinated, and the people fell into anarchy. Several factions were vying for power, and doing anything they could to get their hands on technology. At the time, a Federation research team was on the planet, and were considered prime targets of the rebels. Starfleet decided to send the USS Enterprise-D to retrieve them.
Like all species, some Tilonians succumb to psychological problems, and are sent to the Tilonus Institute for Mental Disorders to combat this. The Enterprise-D's Commander William T. Riker, while trying to retrieve the scientists, was captured by rebels and mind probed for information at the Institute. (TNG: "Frame of Mind")