Expert manuals play an increasingly important role on the development of international law into new areas and environments. While not formally a source of law themselves, their influence on those involved in the development and construction of more formal sources of law takes place on multiple levels and in a variety of ways. This contribution explores some of the different elements that effect the influence these manuals wield. The selection of experts, their qualifications, diversity and ability to adapt to new environments and technological domains is a core components of this influence. Likewise the methodology used by the manual, the attempt to separate lex lata from lex ferenda and the method of representing differing views within the group will all have an impact on its subsequent influence. Manuals will continue to play a valuable role in the development of international law, by influencing the decision of policy makers, treaty negotiators and others, particularly in an era characterised by lack of agreement between states and stagnation of formal law making processes in emerging domains.