This paper describes a tool for supporting grammar development in those linguistic frameworks which employ some constraint-based formalism, such as LFG (Lexical Functional Grammar), HPSG (Head-Driven Phrase Structure Grammar), FUG (Functional Unification Grammar) and CUG (Categorial Unification Grammar). These approaches have in common that all or at least a substantial part of the grammar (such as rules, lexical entries, node labels etc.) is represented as sets of attribute-value pairs. In LISP or Prolog the structures can be internally represented as lists, but it is much more convenient and sometimes even indispensable to use graphical representations when developing grammars. During grammar processing, feature structures can become quite large (up to several thousand nodes), such that a customized view of the feature structure, which allows to selectively focus on relevant parts, becomes essential. Fegramed provides a fully interactive editor for developing, maintaining and viewing feature structures. It is a tool that is built to cope with the complexity of feature structures in grammar development and use.