Iran was the first state that recognized Pakistan following its independence in August 1947. The Pakistani government reciprocated the act in February 1979, when Islamabad pioneered in recognizing the Islamic Republic of Iran subsequent to the victory of the Islamic Revolution. However, despite the aforementioned, bilateral ties between the two neighbors have undergone immense ups and downs during the past six decades. The victory of the Islamic Revolution in Iran in 1979 put an end to the relations between Iran and Pakistan under the Pahlavi regime, opening a new chapter in Islamabad-Tehran ties that featured a vast spectrum of competition and cooperation. Studying the ties between the two states during the past three decades, this essay discusses and explains the five fixed elements of ideology and religion; identity based on ethnicity; religious extremism; developments in Afghanistan, and the nature of ties between India and Pakistan with a special focus on Kashmir and U.S. policies as principal and invariant variables influencing Iran-Pakistan ties. This article aims at identifying and explaining principle and fixed variables influencing the bilateral ties between Iran and Pakistan at the domestic, regional and international levels. The objective of this research is thus to bring about a better and more comprehensive understanding of existing ups and downs in the links between the two states within the framework of the Southwest Asian sub-system.