An arms race is a competition between two or more countries for military supremacy. Each party competes to produce superior numbers of weapons, larger armies, or superior military technology in a technological escalation.

Most arms races have occurred in the modern era. One of the first arms races occurred in the pre-World War I era from the 1890s to 1914 where the five great powers of Europe (Germany, Austria-Hungary, the Russian Empire, France, and the United Kingdom) were locked in an all-out military buildup, ranging from land armies, conscription, and artillery to battleships and competition between each country's mobilization speed. France reached a mobilization speed of just three days. At one point, it was estimated Germany could become fully war ready in only two days. Perhaps the best known arms race of this era was between the British Empire, and the German Empire, when the Royal Navy and the High Seas Fleet spent millions on the development and construction of Dreadnoughts and other types of warships.

One significant recent example was the race to develop more and better nuclear weapons during the Cold War (see: nuclear arms race). Carl Sagan once famously described the overkill in this arms race with the analogy of "two men standing waist deep in gasoline; one with three matches, the other with five." The Soviet Union devoted their command economy to the arms race, but were ultimately defeated by the demands on their economy of matching the United States' greater spending power.

The term "arms race" is used generically to describe any competition where there is no absolute goal, only the relative goal of staying ahead of the other competitors. Evolutionary arms races are common occurrences, e.g. predators evolving more effective means to catch prey while their prey evolves more effective means of evasion. This is sometimes called the Red Queen effect. In addition to predators, parasites can force their hosts into an arms race.

In technology, there are close analogues of the arms races between parasites and hosts, such as the arms race between computer virus writers and anti-virus software writers, or spammers against Internet Service Providers and E-mail software writers.

Lewis Fry Richardson made an arms race model, trying to retrodict World War I, where he showed how two countries would go to war if more money was spent in the arms race than in trade. There is an interesting economic idea here, but since the model was made after the war, it did not have the prophetic power it might have had.