Graphs can be used to convey information about relationshipsin many applications. State transition diagrams, PERT/CPMcharts, and entity-relationship diagrams are a few examplesof many applications involving graphs. Typically, nodes inthe graph represent items in the application and the edgesrepresent the relationships among these items. A grapheditor is an interactive tool that presents a graph to theuser pictorially and allows the user to edit the graph.This monograph presents the design of an extendible grapheditor, which is a graph editor that can be adapted easilyto many different application areas. Several fundamental andrecurring problem areas associated with graph editors areinvestigated and a solution is proposed for each. The topicsinvestigated are:- Graph layout: a layout constraint mechanism is presentedwhich can easily be combined with various graph layoutalgorithms.- Graphical abstraction: a novel clustering techniquecallededge concentration is presented which can reduce theapparent complexity of the graph.- Persistence: the graph structures produced by theeditorcan be kept in long-term storage using a standardized,external format for graphs.- Extendibility: the design of the proposed graph editormakes it easy to adapt to various applications.To demonstrate their feasibility, the proposed solutionshave been incorporated into EDGE, an extendible graph editorprotoype.