The M109 has been the most successful family of self-propelled guns produced by the US, becoming the de facto standard 155-mm SPG for the US and NATO's mechanized units during the Cold War and beyond. The M109 was developed concurrently with the smaller-caliber M108 which it shared many features including the same all-welded aluminum hull, turret, and suspension, as well as the same turborcharged diesel engine. Armament has always consisted in 155-mm guns of varying caliber, beginning with the short-barreled (L/23) M126 in the original production version (which could fire up to three rounds per minute) up to the long-barreled M284 (L/39) in the most recent variants like the M109A6 Paladin. This has given the M109 vastly expanded range, from the 14.7 km of the original to as high as 30 km today using rocket-assisted projectiles and 40 km with more advanced ammunition currently in development. Ammunition is loaded via twin doors at the rear of the turret while stability is provided by a spade at the rear of the hull. Defensive armament includes one 7.62-mm or 12.7-mm machine gun, and later versions feature NBC protection as well. Optional amphibious capability is also offered via flotation bags attached to the front and sides. The M109 quickly became the standard SPG of the US Army and obtained immediate export orders (including license production) from many NATO allies including Belgium, Germany, Greece, Italy, Netherlands, and the UK, in addition to non-NATO clients such as Brazil, Egypt, Israel, Pakistan, South Korea, and Switzerland. They have seen extensive combat including by the US in the Vietnam War, Gulf War, and Iraq War, as well as by other users in numerous Middle Eastern conflicts. A small number donated by Norway have also been used by Ukraine in its war with Russia. Many remain in service, although they have since been phased out or complemented by newer SPGs such as the AS-90 (UK), PzH 2000 (Germany), and K9 (South Korea). The absence of a replacement thus far in the 21st century means it will remain in US service for quite some time.

Development of a new self-propelled howitzer known as the T196 began in 1952 with the first prototype completed in 1959 with a gasoline engine and later diesel engine in the T196E1. The latter eventually became the production M109. The first major upgrade became known as the M109A1 and featured a slightly longer M185 gun with a prominent fume extractor, which expanded range to 18 km (export versions were designated M109A1B). Mechanical improvements and additional stowage resulted in the M109A2, while the M109A3 was a further incremental mechanical and equipment upgrade to existing M109A1s. M109A2s brought to this standard were designated M109A4 with the addition of NBC protection. The M109A5 was a major upgrade which introduced a longer-barreled M284 in a new mount, an improved engine and transmission. Further improvements led to the M109A6 Paladin (the only variant which was formally named), which featured a redesigned turret, GPS guidance and automatic fire control, digital communications, as well as a host of additional electronic and equipment upgrades. The newest version is the M109A7 which incorporates common chassis components from the M2/M3 Bradley to facilitate logistics and maintenance. Foreign variants are mostly based on US versions, often with local equipment changes. German variants were known as the M109G and M109A3G and had extended range thanks to modifications from Rheinmetall. The Italian M109L featured armament designed by OTO Melara. Swiss versions were known as the Panzerhaubitze 66 (M109), 74, 79, and 88 (M109A1B), with a newer version armed with a locally-designed L/47 caliber gun is known as the KAWEST. South Korean-built variants were known as the K55 (M109A2) and feature extensive use of domestic components. These were upgraded into the K55A1 which was intended to keep them competitive following the introduction of the K9.