This 1991 yearbook continues SIPRI's review of the latest developments in nuclear weapons, nuclear explosions, world military expenditure, the international arms trade and arms production, chemical and biological weapons, the military use of outer space, the proliferation of ballistic missile technology, armed conflicts in 1990, US-Soviet nuclear arms control, and conventional arms control in Europe. The study also outlines the situation in the changing Europe of 1990, and considers the new security structures for Europe which will be defined in the early years of the decade.