The Saab Viggen (Thunderbolt) or Saab 37 is a Swedish fighter aircraft manufactured between 1971 and 1990 to replace the Saab Lansen.

Design of the Viggen began in the early 1960s with the aim of producing a robust single-engined fighter with excellent STOL performance that could be operated from improvised facilities such as roadways in the event of nuclear attacks on major installations. Other requirements included supersonic ability at low level, Mach 2 performance at altitude, and the ability to make short landings at low angles of attack (to avoid damaging improvised runways).

To meet these tough design goals, Saab selected a radical canard delta configuration: essentially a conventional delta wing (like the Mirage III or the Convair Delta Dart) with small, high-set foreplanes. (Carnards have since become common in fighter aircraft, notably with the Eurofighter Typhoon and the IAI Kfir.)

(A 'Vigg' is also a tufted duck 'Aythya fuligula', found in Sweden. 'Canard' is French for duck.)

Power is provided by a single Volvo Flygmotor RM8B turbofan - essentially a licence-built version of the same Pratt & Whitney JT8D that powered commercial airliners of the '60s like the DC-9 and the 727 but with an afterburner.

The Viggen entered service with the Swedish armed forces with a ground attack optimised version in 1971. After 110 of the original AJ 37s and 18 Sk 37 two-seat trainers were delivered, 26 of the SF 37 reconnaissance version, and another 26 of the SH 37 maritime reconnaissance and strike version were built.

Although the Viggen was offered for sale worldwide, and regarded as a very competent aircraft, no export sales eventuated. Reasons advanced to explain Saab's failure to sell a competitively priced and well-respected aircraft include the Swedish government's relatively strict controls on arms exports to undemocratic countries, potential customers' doubts about continuity of support and supply of spare parts in the event of a conflict disapproved of by Sweden, and strong diplomatic pressure on buyers larger nations (the United States in particular).

The final Viggen variant was the JA 37 interceptor. The last of 149 JA 37s was delivered in 1990. Various upgrades have been performed over the years, mainly to cockpit equipment, weapons and sensor fit, but the Viggen is now being phased out in favor of the 5th generation Gripen.