The North Suburban Library System decided that the areas within its boundaries which are outside the taxed district/area supporting an existing library should be served by extending the service areas of the existing municipal, township, and district libraries, either through contract or by employing the provisions of present library district law. In order to guide the expansion so that it results in maximum accommodation to potential library users, the System engaged a professional research organization to design the allocation of unserved areas to existing library facilities in a way that reflects the natural orientation and convenience of the population. The configurations resulting from such an allocation are termed "natural library service zones" - zones within which it may be presumed that residents are oriented toward the library within the zone rather than toward any other library. Part I of this report describes the working concepts and methodology used in deriving the natural service zones. Part II, in the form of an annotated map, depicts the boundaries of the zones. Appendixes include: (1) details of a survey of 300 library users, conducted to verify the applicability of conventional retail location theory, (2) information on the delineation of retail trade areas, and (3) data from a survey of interstitial areas. (Author/JB)