This chapter presents and applies an original framework – type theory – to identify a prohibited ‘use of force’ between States under article 2(4) of the UN Charter, focusing on the meaning of ‘use of force’ and the contextual element of ‘international relations’. The theory of ‘type’ is firstly set out before explaining how it applies to the prohibition of the use of force. This chapter argues that a prohibited ‘use of force’ between States is characterised by a basket of elements, not all of which must be present for an act to meet the definition. Instead, these elements – including certain effects, gravity and intention – are identified and weighed to determine whether the definition of a ‘use of force’ is met. Illustrative examples are given from State practice including targeted killing and excessive or unlawful maritime law enforcement. Finally, this chapter applies the type theory framework to the testing of anti-satellite (ASAT) weapons in outer space to show how it can be applied in novel contexts. Through these case studies, this chapter draws definitive conclusions regarding the definition of a ‘use of force’ and demonstrates how to apply this framework in practice.