In this, the second decade of the twenty-first century, the law relating to conflict is confronted by a number of challenges that this book seeks to identify and to discuss. It was a deliberate decision that the book should cover the whole spectrum of conflict from general war to situations below the armed conflict threshold. The title ‘Conflict Law’ should be seen in that light. Old legal certainties based on a bi-polar system of war and peace have given way to ambiguities as we apply the current, more extensive legal spectrum of conflict to contemporary transnational conflicts involving loosely affiliated armed groups. Gaps in treaty law governing armed conflict seem unlikely to be filled in the short term, so what is the legal status of the numerous writings of Experts that we have seen in recent decades? The Internet offers a new environment in which hostilities can be conducted and for which there are no treaty rules of the game. Technological advance seems likely to produce, in both the real and virtual environments, increasing numbers of automated and, in due course, autonomous weapons that make their own attack decisions, which the machine then implements. How does a body of law written on the implicit premise of human decisionmaking cope with the onward march of the empowered machine? Weapons technology continues to advance at a rapid pace, but states are obliged to apply existing legal principles when determining the legitimacy of the new tools of war. Determining how the rules should be applied to cutting edge technologies, such as autonomous, cyber, nanotechnology and outer space weapons, is going to be an important undertaking. Remote attack techniques that render the attacker invulnerable, effects-based operational thinking that seeks to expand the envelope of permissible targeting, the persistent issues associated with asymmetry and a likely depopulation of the battlefield seem likely to cause some to question deeprooted legal principles. Despite these technological developments, however, people will remain central to the conduct of hostilities, although their roles may change over time and increasing involvement of civilians may become legally problematic.