Klaestron were humanoid, with distinct streak-like protrusions on the side of their head above their ears. Their outer ears consists only of a hole in their head.
The Klaestron Civil War took place in the mid 24th century. Although they had a unilateral extradition treaty with the Federation, the Klaestron were allies with the Cardassians. Treason and murder were punishable by death among the Klaestron. (DS9: "Dax")
Around 2371, the Klaestrons developed a new burn treatment technology. That year, Doctor Julian Bashir visited Klaestron IV to review the new technology noting that it "has to be seen to be believed." (DS9: "Second Skin")