1. For a recent review see: A. Djouadi, D. Schaile, C. Verzegnassi [convenors] et al.: Report of the Working Group ?Extended Gauge Models? in the Proceedings of [2]
2. Proceedings of the Workshop ?e + e ? Collisions at 500 GeV: the physics potential?, P. Zerwas, (ed.) Report DESY 92-123A+B
3. For a review of LEP data and constraints on New Physics see P. Langacker, Summary talk given at XXVII Rencontres de Moriond on Electroweak Interactions and Unified Theories, Les Arcs, France, March 1991, Report UPR-0512T
4. For a review on E6 and for a complete set of references on grand unified groups see, J. Hewett, T.G. Rizzo: Phys. Rep. 183 (1989) 193
5. For a review see: J. Maalampi, M. Roos: Phys. Rep. C186 (1990) 53