The document is the result of a series of advisory and consultants meetings held within the framework of the International Working Group on Nuclear Power Plant Control and Instrumentation in 1989-1990. It provides a basis for assigning functions to men and machines and for achieving a desirable balance. It should be particularly useful to designers of of new systems, where a large number of assignment decisions will have to be identified, taken and documented. In addition, the methodology can be used by utilities for plant modifications and upgrades. The document may also be employed for examining existing assignments in a system since the principles on which the document is based are generally applicable. The document may be useful to those who develop requirement specifications for automation, to technology designers who design automated machines, and to researchers who intend to further refine the function assignment methodology. Refs, figs and tabs.