The United States is concurrently pursuing the goals of reducing the size of its nuclear weapons force -- strategic and non-strategic, deployed and non-deployed -- and of modernising the weapons it continues to possess. Many of the existing systems were deployed 30 to 50 years ago, and the modernisation process can be expected to extend over the next decade or more. This book examines the motivations for reductions and modernisation; reviews a number of historical systems; discusses the current path forward for the United States nuclear force; provides background, developments, and issues of the United States strategic nuclear forces; and discusses a nuclear posture review report.