Borchers H.-J. Quantum Field Theory and Modular Structures (93K, TeX) ABSTRACT. A survey of the use of the Tomita--Takesaki modular theory in the theory of local observables will be given. This contains, in particular, the characterization of chiral field theories in terms of the concept of modular inclusions. Moreover, it contains the reconstruction of the translation--goup fulfilling spectrum condition out of the modular conjugations of the algebras associated with wedge--domains. Finally examples will be given wich show that neither duality nor wedge--duality are consequences of the standard axioms. These examples are constructed by using the modular theory.